Management of Fungal Plant Pathogens
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Management of Fungal
Plant Pathogens
Edited by
Arun Arya
Professor and Head, Department of Botany and
Coordinator Environment Science Programme, Faculty of Science
The Maharaja Sayajirao University of Baroda,
Vadodara, India
and
Analía Edith Perelló
Assistant Professor and Research Scientist, CONICET - CIDEFI,
and Coordinator MSc Vegetal Protection Programme, Plant Pathology,
Facultad de Ciencias Agrarias y Forestales, Universidad Nacional
de La Plata, Provincia de Buenos Aires, Argentina
CABI is a trading name of CAB International
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Library of Congress Cataloging-in-Publication Data
Management of fungal plant pathogens / edited by Arun Arya,
Analía Edith Perelló.
p. cm.
Includes bibliographical references and index.
ISBN 978-1-84593-603-7 (alk. paper)
1. Fungal diseases of plants. 2. Phytopathogenic fungi–Control.
3. Plant-pathogen relationships. I. Arya, Arun. II. Perelló,
Analía Edith. III. title.
SB733.M36 2010
632'.4–dc22
2009023395
ISBN-13: 978 1 84593 603 7
Typeset by AMA Dataset, Preston, UK.
Printed and bound in the UK by the MPG Books Group.
Contents
Contributors
Preface
viii
xi
PART I: BOTANICALS IN FUNGAL PEST MANAGEMENT
1
Recent Advances in the Management of Fungal Pathogens of Fruit Crops
Arun Arya
2
Botanicals in Agricultural Pest Management
Ashok Kumar, Priyanka Singh and N.K. Dubey
3
Deleterious Effects of Fungi on Postharvest Crops and Their
Management Strategies
A.O. Ogaraku
28
Exploitation of Botanicals in the Management of Phytopathogenic
and Storage Fungi
Pramila Tripathi and A.K. Shukla
36
Use of Plant Extracts as Natural Fungicides in the Management of
Seedborne Diseases
Gustavo Dal Bello and Marina Sisterna
51
4
5
3
14
PART II: DISEASE CONTROL THROUGH RESISTANCE
6
Resistance to Septoria Leaf Blotch in Wheat
María R. Simón
69
7
Barley and Wheat Resistance Genes for Fusarium Head Blight
S.A. Stenglein and W.J. Rogers
78
v
vi
8
Contents
Sustainable Management of Rice Blast (Magnaporthe grisea
(Hebert) Barr): 50 Years of Research Progress in Molecular Biology
S. Nandy, N. Mandal, P.K. Bhowmik, M.A. Khan and S.K. Basu
92
PART III: BIOLOGICAL CONTROL MECHANISMS
9
10
11
Postharvest Technology – Yeast as Biocontrol Agents: Progress,
Problems and Prospects
Neeta Sharma and Pallavi Awasthi
109
Biological Control of Plant Diseases: An Overview and
the Trichoderma System as Biocontrol Agents
Abhishek Tripathi, Neeta Sharma and Nidhi Tripathi
121
Physiological Specialization of Ustilaginales (Smut) of
Genera Bromus, Zea and Triticum in Argentina
Marta M. Astiz Gassó and María del C. Molina
138
PART IV: ENDOPHYTES IN PLANT DISEASE CONTROL
12
13
14
15
Status and Progress of Research in Endophytes from
Agricultural Crops in Argentina
Silvina Larrán and Cecilia Mónaco
149
Effect of Tillage Systems on the Arbuscular Mycorrhizal Fungi
Propagule Bank in Soils
Santiago Schalamuk and Marta N. Cabello
162
Mechanism of Action in Arbuscular Mycorrhizal Symbionts
to Control Fungal Diseases
Arun Arya, Chitra Arya and Renu Misra
171
Role of Fungal Endophytes in Plant Protection
S.K. Gond, V.C. Verma, A. Mishra, A. Kumar and R.N. Kharwar
183
PART V: MANAGING FUNGAL PATHOGENS CAUSING LEAF DAMAGE
16
The Rust Fungi: Systematics, Diseases and Their Management
M.S. Patil and Anjali Patil
201
17
Etiology, Epidemiology and Management of Fungal Diseases of Sugarcane
Ayman M.H. Esh
217
18
New and Emerging Fungal Pathogens Associated with Leaf Blight
Symptoms on Wheat (Triticum aestivum) in Argentina
Analía Edith Perelló
231
Diseases of Fenugreek (Trigonella foenum-graecum L.) and Their Control
Measures, with Special Emphasis on Fungal Diseases
S.N. Acharya, J.E. Thomas, R. Prasad and S.K. Basu
245
19
Contents
vii
20
Fungal Diseases of Oilseed Crops and Their Management
S.S. Adiver and Kumari
263
21
Occurrence of Pyrenophora tritici-repentis Causing Tan Spot in Argentina
M.V. Moreno and A.E. Perelló
275
22
Epidemiological Studies on Septoria Leaf Blotch of Wheat in Argentina
Cristina A. Cordo
291
PART VI: ALTERNATIVE CONTROL STRATEGIES
23
24
25
Review of Thecaphora amaranthicola M. Piepenbr., Causal Agent
of Smut on Amaranthus mantegazzianus Pass.
M.C.I. Noelting, M.C. Sandoval, M.M.A. Gassó and M.C. Molina
311
Population Biology and Management Strategies of Phytophthora sojae
Causing Phytophthora Root and Stem Rots of Soybean
Shuzhen Zhang and Allen G. Xue
318
Management of Fungal Pathogens – A Prerequisite for Maintenance of
Seed Quality During Storage
Anuja Gupta
329
26
Controlling Root and Butt Rot Diseases in Alpine European Forests
Paolo Gonthier
345
27
Some Important Fungal Diseases and Their Impact on Wheat Production
Aakash Goyal and Rajib Prasad
362
Index
The colour plate section can be found following page 50.
375
Contributors
Acharya, S.N., Agriculture and Agri-Food Canada Research Centre, Lethbridge, AB,
Canada T1J 4B1
Adiver, S.S., Oilseeds Scheme, Main Agricultural Research Station, University of Agricultural
Sciences, Dharwad 580 005, Karnataka, India (shivaputra_adiver@rediffmail.com)
Arya, Arun, Department of Botany, Faculty of Science, The Maharaja Sayajirao University
of Baroda, Vadodara 390002, India (aryaarunarya@rediffmail.com)
Arya, Chitra, Department of Botany, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, India (caarya@yahoo.co.in)
Astiz Gassó, Marta M., Instituto Fitotécnico Santa Catalina (IFSC), Facultad de Ciencias
Agrarias y Forestales, Universidad Nacional de La Plata, CC 4, 1836 Llavallol, Buenos
Aires, Argentina (astizgasso@yahoo.com.ar)
Awasthi, Pallavi, Mycology and Plant Pathology Division, Department of Botany, University of Lucknow, Lucknow 226007, India
Basu, S.K., Department of Biological Sciences, University of Lethbridge, Lethbridge, AB,
Canada T1K 3M4 (saikat.basu@uleth.ca)
Bhowmik, P.K., Bioproducts and Bioprocesses, Lethbridge Research Center, Agriculture
and Agri-Food Canada, Lethbridge, AB Canada T1J 4B1
Cabello, Marta N., Comisión de Investigaciones Científicas de la Provincia de Buenos Aires
(CICBA) – Instituto de Botánica Spegazzini, Calle 53 N° 577, 1900 La Plata, Argentina
(mcabello@museo.fcnym.unlp.edu.ar)
Cordo, Cristina A., Comisión de Investigaciones Científicas de la Provincia de Buenos
Aires, Centro de Investigaciones de Fitopatología (CIDEFI) – Facultad de Ciencias Agrarias y Forestales, 60 y 119, (1900) La Plata, Argentina (cristcordo@hotmail.com)
Dal Bello, Gustavo, Centro de Investigaciones de Fitopatología (CIDEFI), Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, 60 y 119, CC 31, 1900 La
Plata, Argentina (dalbello@speedy.com.ar)
Dubey, N.K., Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi
221005, India (nkdubey2@rediffmail.com)
Esh, Ayman M.H., Biotechnology and Tissue Culture Laboratories, Sugar Crops Research
Institute, Agricultural Research Center, Giza, Egypt (aymanesh@gmail.com)
Gond, S.K., Mycopathology and Microbial Technology Laboratory, Centre of Advanced
Study in Botany, Banaras Hindu University, Varanasi 221005, India
viii
Contributors
ix
Gonthier, Paolo, Department of Exploitation and Protection of Agricultural and Forestry
Resources (DIVAPRA), Plant and Forest Pathology, University of Torino, Via L. da Vinci,
44, I-10095 Grugliasco (TO), Italy (paolo.gonthier@unito.it)
Goyal, Aakash, Agriculture and Agri-Food Canada, Lethbridge Research Center, Lethbridge
AB-T1J4B1, Canada (akgroyal@gmail.com)
Gupta, Anuja, Indian Agricultural Research Institute, Regional Station, Karnal – 132 001,
Haryana, India (agupta_2005@yahoo.com)
Khan, M.A., Department of Weed Science, NWFP Agricultural University, Peshawar, NWFP,
Pakistan 25130
Kharwar, R.N., Mycopathology and Microbial Technology Laboratory, Centre of Advanced Study
in Botany, Banaras Hindu University, Varanasi 221005, India (rnkharwar@yahoo.com)
Kumar, Ashok, Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi
221005, India
Kumari, Oilseeds Scheme, Main Agricultural Research Station, University of Agricultural
Sciences, Dharwad 580 005, Karnataka, India
Larrán, Silvina, Centro de Investigaciones de Fitopatología (CIDEFI), Facultad de Ciencias
Agrarias y Forestales, Universidad Nacional de La Plata, 60 y 119, CC 31, 1900 La Plata,
Argentina
Mandal N., Bidhan Chandra Krishi Vishavidalay, Nadia, WB, India 741252
Mishra, A., Mycopathology and Microbial Technology Laboratory, Centre of Advanced
Study in Botany, Banaras Hindu University, Varanasi 221005, India
Misra, Renu, Department of Botany, Faculty of Science, The Maharaja Sayajirao University
of Baroda, Vadodara 390002, India
Molina, María del C., Consejo de Investigaciones Científicas y Técnicas (CONICET),
Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, CC 4,
1836 Llavallol, Buenos Aires, Argentina
Mónaco, Cecilia, Centro de Investigaciones de Fitopatología (CIDEFI), Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, 60 y 119, CC 31, 1900 La
Plata, Argentina (cecilia.monaco7@gmail.com)
Moreno, M.V., CONICET – Facultad de Agronomía de Azul, Universidad Nacional del
Centro de la Provincia de Buenos Aires, República de Italia No. 780, Azul CP 7300, Buenos Aires, Argentina (morevir@yahoo.com.ar)
Nandy, S., Bioproducts and Bioprocesses, Lethbridge Research Center, Agriculture and
Agri-Food Canada, Lethbridge, AB Canada T1J 4B1
Noelting, M.C.I., Instituto Fitotécnico de Santa Catalina, Facultad de Ciencias Agrarias y
Forestales, Universidad Nacional de La Plata, Garibaldi 3400, Llavallol 1836 CC 4 Buenos Aires, Argentina (mcnoelting@hotmail.com)
Ogaraku, A.O., Plant Science and Biotechnology Unit, Department of Biological Sciences,
Nasarawa State University, PMB 1022, Keffi, Nasarawa State, Nigeria (ogara006@yahoo.
com)
Patil, Anjali, Department of Botany, Rajaram College, Kolhapur 416004 (M.S.), India
(dhirajanj@gmail.com)
Patil, M.S., Department of Botany, Shivaji University, Kolhapur (M.S.), India
Perelló, Analía Edith, CIDEFI (Centro de Investigaciones de Fitopatología) – CONICET
(Consejo Nacional de Investigaciones Científicas y Técnicas), Facultad de Ciencias
Agrarias y Forestales de la Universidad Nacional de La Plata, La Plata, Provincia de
Buenos Aires, Argentina (anaperello@yahoo.com.ar)
Prasad, Rajib, Agriculture and Agri-Food Canada, Lethbridge Research Center, Lethbridge
AB-T1J4B1, Canada
Rogers, W.J., Laboratorio de Biología Funcional y Biotecnología (BIOLAB), Facultad de
Agronomía, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN),
x
Contributors
Av. República de Italia # 780 (CC 47), (7300) Azul, Buenos Aires, Argentina; FIBA –
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
Sandoval, M.C., Facultad de Ciencias Agrarias, UNLZ, Ruta 4 Km 2 Llavallol, Buenos Aires,
Argentina
Schalamuk, Santiago, CONICET – Centro de Investigaciones de Fitopatología (CIDEFI) y
Cerealicultura, Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La
Plata, 60 y 119, CC 31, 1900 La Plata, Argentina (sschala@yahoo.com.ar)
Sharma, Neeta, Mycology and Plant Pathology Division, Department of Botany, University
of Lucknow, Lucknow 226007, India (dr_neeta_sharma2003@yahoo.com)
Shukla, A.K., Department of Botany, Rajiv Gandhi University, Rono Hills, Itanagar 791
112, India
Singh, Priyanka, Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi
221005, India
Simón, María R., Cerealicultura, Facultad de Ciencias Agrarias y Forestales, Universidad
Nacional de La Plata, 60 y 119, CC 31, 1900 La Plata, Argentina (mrsimon@agro.unlp.
edu.ar)
Sisterna, Marina, Centro de Investigaciones de Fitopatología (CIDEFI), Facultad de Ciencias
Agrarias y Forestales, Universidad Nacional de La Plata, 60 y 119, CC 31, 1900 La Plata,
Argentina (mnsisterna@infovia.com.ar)
Stenglein, S.A., Laboratorio de Biología Funcional y Biotecnología (BIOLAB), Facultad de
Agronomía, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN), Av.
República de Italia # 780 (CC 47), (7300) Azul, Buenos Aires, Argentina; FIBA – Consejo
Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina (stenglein@
faa.com.unicen.edu.ar)
Thomas, J.E., Department of Biological Sciences, University of Lethbridge, Lethbridge, AB,
Canada T1K 3M4
Tripathi, Abhishek, Department of Bioscience and Biotechnology, Banasthali University,
PO Banasthali Vidyapith, 304022 Rajasthan, India (abhitri77@yahoo.com)
Tripathi, Nidhi, Department of Bioscience and Biotechnology, Banasthali University, PO
Banasthali Vidyapith, 304022 Rajasthan, India
Tripathi, Pramila, Department of Botany, D.A.V.-P.G. College, Kanpur 208001 (U.P.), India
(pramilatripathi_bhu@rediffmail.com)
Verma, V.C., Mycopathology and Microbial Technology Laboratory, Centre of Advanced
Study in Botany, Banaras Hindu University, Varanasi 221005, India
Xue, Allen G., Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food
Canada, 960 Carling Ave., Ottawa, Ontario, Canada, K1A 0C6 (axue@agr.gc.ca)
Zhang, Shuzhen, Soybean Research Institute, Key Laboratory of Soybean Biology of Chinese
Education Ministry, Northeast Agricultural University, Harbin, Heilongjiang, China, 150030
Preface
Oldest life forms have been reported from the North Pole Dome area of Western Australia,
which dates back 3556 million years. Non-septate mycelium remains of Eomycetopsis
robusta were recovered from late Precambrian chert of Australia. Having appeared first on
planet Earth, microbes have immense potential to influence all other life forms. Plant diseases have caused epidemics and have had a profound influence on wars, famine and the
changing economy. Microbes including fungi need no introduction to common man; they
are progressive, ever changing and evolving in their own way, so they are capable of adapting to every condition of life. The French biochemist, Louis Pasteur, once said, ‘The role of
the infinitely small is infinitely large.’
Potentially immortal fungi spread their tentacles in 1845, when potato late blight fungus caused havoc in Ireland. Soon after, Plasmopara viticola threatened the wine industry
in France. First reported in 1819 in Sweden, apple scab disease caused by Venturia inaequalis threatened apple cultivation in the Kashmir Valley in India in 1973. Panama disease of
banana, wilt diseases of pigeon pea, castor and guava and smut and rust of cereals are some
other serious fungal diseases. The chance discovery of Bordeaux mixture by P.A. Millardet
in France paved the way to the chemical control of plant diseases. Phytopathologists are
confronted by a volley of challenges in the wake of a resurgence of new diseases and the
obligation to fulfil international trade agreements. We have to protect the environment and
at the same time ensure the safety and security of farmers in the field by making a concentrated effort to minimize crop losses due to fungi and other microbes.
This book provides an overview of our current knowledge of some plant–pathogen
interactions in economically important crops, emphasizing the importance of pathogenic
fungi on fruits, cereals, postharvest crops and the establishment of plant diseases and drawing together fundamental new information on their management strategies based on conventional and eco-friendly methods, with an emphasis on the use of microorganisms and
various biotechnological aspects of agriculture, which could lead to sustainability in modern agriculture.
The book examines the role of microbes in growth promotion, as bioprotectors and
bioremediators, and presents practical strategies for using microbes in sustainable agriculture. In addition, the use of botanicals vis-à-vis chemical pesticides has also been reviewed.
Contributions on new research fields such as mycorrhizae and endophytes have been
xi
xii
Preface
included. The book also examines in different chapters host–pathogen interactions in the
light of the new tools and techniques of molecular biology and genetics.
Dr Arya expresses his deep sense of indebtedness and admiration to the late Dr S.N.
Bhargava and to Professor Bihari Lal, ex Head of the Department of Botany, University of
Allahabad, who taught him his first lessons in plant pathology at the University of Allahabad. He is grateful to his father, the late Shri O.P. Arya, for inspiring him to write about the
management of plant diseases and pests, which has proved most useful to plant growers.
He honours his grandfather, Baba Shankaranand, who fed him with sweet mangoes during
his childhood and who motivated him to love plants and to learn how to nurture them and
research into new and improved varieties.
We are grateful to the entire staff of our institutions and the cooperation and collaborative efforts of the plant pathology experts of Argentina (Universidad Nacional de La Plata,
Universidad Nacional de Lomas de Zamora, Universidad Nacional del Centro) and India
(Botany Department, The Maharaja Sayajirao University of Baroda), who made this book
possible.
We thank all those who have contributed their valuable articles to this volume and are
sure that the present work, which consists of 27 different chapters written by learned
experts in the field, will be immensely useful to postgraduate students, researchers, academics, progressive farmers and practising horticulturists, as well as those involved in the
various agro-industries. We are hopeful that the available knowledge in the field, newer
technologies and disease-resistant varieties will be used in different parts of the world and
that ultimately the plant disease scenario will change. All appreciations and good wishes
are extended to the members of the CABI team, particularly Ms. Sarah Mellor, for helpful
discussions and skilled assistance in the reviewing of the manuscripts, and also for helping
us in various ways to accomplish this project satisfactorily in the stipulated time. And also
for the cooperation and collaborative effort of the Plant Pathology experts that made this
book possible.
Arun Arya
Analia Edith Perelló
Part I
Botanicals in Fungal
Pest Management
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1
Recent Advances in the Management
of Fungal Pathogens of Fruit Crops
Arun Arya
Department of Botany, Faculty of Science, The Maharaja Sayajirao University
of Baroda, Vadodara, India
Abstract
Fruits constitute a rich source of sugars, vitamins, minerals and medicinally important compounds
like flavonoids, which prevent cancer and cardiovascular diseases. These are eaten as a dessert or
processed into jams, jellies, ice creams and drinks; grapes are dried to make raisins. The science of
protecting fruit crops began with the discovery of Bordeaux mixture by P.A. Millardet in France. But
still we have yet to find many new techniques and fungicide formulations to control diseases; such as
bunch rot of grapes (Botrytis cinerea), apple scab (Venturia inaequalis), wilt of guava (Fusarium
solani), Panama wilt of banana (F. cubense), mango malformation (F. moniliforme), blue mould of
citrus (Penicillium citrinum) and anthracnose of papaya (Colletotrichum papayae), etc. Losses from
postharvest fruit diseases range from 1 to 20% in the USA and from 10 to 40% in India. The pathogens
have developed resistance against various fungicides and the postharvest phase is minimized. Alternative strategies like the use of biocontrol methods and the application of botanicals have been tried. A
large number of plants are screened for the presence of effective secondary metabolites. Integrated pest
management, using improved cultural practices (pruning methods to control Botrytis bunch rot in
grapes), the use of solarization (in strawberries), the application of growth hormone (NAA in the case
of mango malformation), along with minimum dosage of fungicides, are recommended to control
various fruit diseases.
The world fruit market is expanding; we are more concerned about human nutrition now, but at
the same time serious enough to protect the environment from pollution. The economics of a success
story will have to revolve around the use of various cutting-edge technologies and, at the same time,
the use of simpler and more effective methods acceptable to fruit growers. Biotechnologists have tried
to enhance the activity of biocontrol agents; at the same time, efforts are being made for genetic transformation involving molecular breeding. This technology involves intimate knowledge of the gene,
regulatory components and gene functional environment (i.e. the domain where the gene is located).
Once an understanding of the molecular basis of genes involved in resistance has been achieved, we
will be able to isolate the alleles of those genes and their inclusion will lead to transformed, diseasefree plants.
Introduction
Fruits constitute an important component
of our daily diet. The use of dates, fig, mango
and grapes is mentioned in ancient texts.
Taken either as a dessert or processed, the
nutritional value of fruits depends chiefly
on the quality and concentration of sugars,
vitamins and other essential minerals. Plants
suffer with a number of diseases and pests
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
3
4
A. Arya
during their growth phase. Fungi not only
blemish, disfigure or cause rot to a number
of fruits but also reduce their market value
(Arya, 2004). Realizing the importance of
postharvest diseases, Stevens and Stevens
(1952) mentioned, ‘of all losses caused by
plant disease those that occur after harvest
are the most costly, whether measured in
monetary terms or in man hours’. India is
the second most important fruit producing
country of the world. It produces the highest quantity of mango, while the productivity of grapes in India at 56 t/ha is a world
record. The export of mango increased in
the nineties from 25,000 to 44,000 t (a 25%
share of world trade) (Neelam, 1993). Fruit
production is 49 Mt (Arya, 2004).
The fruit growing industry has developed a lot, overcoming the hurdles of biotic
and abiotic stresses. The industry needs a
comprehensive strategy to face the challenges and opportunities of a global economy. Lewis (1985) stated that ‘a few key
discoveries have led to a breakthrough in
our understanding of the biological genome
and our ability to alter it, which may equal
in significance the development of nuclear
energy in the Physical Sciences.’
Fungal Infection of
Fruits and Fruit Trees
Brown rot of citrus fruit is caused in the
orchard by Phytophthora citrophthora. Many
fungi that penetrate the host in the field
cause quiescent infection. The grey mould
in strawberries is caused by B. cinerea. The
Botrytis spores with which the strawberry
field is filled during bloom can germinate in
a drop of water on the petal or other parts of
the flower and later penetrate the senescenced parts of the flower into the edge of
the receptacle of the strawberry, where they
develop a dormant mycelium. During ripening and storage, as the resistance of the fruit
to the pathogen decreases, the preliminary
mycelium enters an active stage and decay
develops (Powelson, 1960; Jarvis, 1962). Postharvest pathogens can be divided, according
to the timing of their penetration of the host,
into those that penetrate the fruits while
still in the field, but develop in their tissues
only after harvest, during storage or marketing, and those that initiate penetration during
or after harvest. Symptoms in stylar end rot of
guava caused by Phomopsis psidii become
more prominent during storage (Arya, 1983).
Verhoeff (1974) describes how quiescent
infection is established in young fruits:
1. Shortage of adequate substances in
young fruits.
2. The incapability of the pathogen to produce cell wall degrading enzymes in the
young fruit.
3. The presence of antifungal compounds.
4. The accumulation of phytoalexins (Swinburne, 1983).
The first theory claims that young unripe
fruit does not provide the pathogen with the
nutrition and energy required for its development. The artificial increase of the sugar level
in apple was achieved by the use of a chemical such as 2,4-dinitrophenol on the fruit. It
accelerated the decay caused by Botryosphaeria ribis (Sitterly and Shay, 1960). It has been
found that antifungal compounds become
toxic in the presence of sugars.
The second theory suggests that the
unripe fruit does not supply the pathogen
with compounds that induce activity in cell
wall degrading pectolytic enzymes.
The third and fourth theories point to a
relation between the formation of antifungal
compounds in the young tissues and the
creation of quiescent infections. Chemicals
such as 3,4-dihydroxy benzaldehyde have
proven fungistatic activity in the green
banana fruit. In unripe avocado fruit, a link
has been established between the presence of
a diene and monoene antifungal compounds
in the fruit rind and the quiescent infection
of C. gloeosporioides in such a fruit. The
reduction in the concentration of the diene
probably results from lipoxygenase enzymatic activity that increases as ripening
progresses and the fruit softens (Prusky
et al., 1982, 1985). The dormant state of
Alternaria alternata in young mango fruits
has been attributed to the presence of two
antifungal resorcinols in the unripe fruit
rind (Droby et al., 1986).
Recent Advances in the Management of Fungal Pathogens of Fruit Crops
Recent Advances in the Management
of Fungal Pathogens
Cultural practices
Initial infection of most temperate fruits is
carried from the orchard; therefore, preharvest cultural practices, if adopted, considerably reduce postharvest diseases during
transit and storage. Strict orchard hygiene
and maintenance of tree vigour is recommended to reduce losses from Botryosphaeria rot of apple. Pezicula malicorticis and
Nectria galligena infection in apple start
from cankered portions. The removal of
dead and senile plant parts and canker portions helps to reduce the incidence of many
postharvest diseases. The incidence of many
rots may also be reduced if the rotted fruits
are frequently collected and dumped in a
deep trench and later covered with a thick
layer of soil to prevent the dissemination of
their spores. If such rotted fruits are destroyed
by burning some distance away from the
orchard, this also helps to reduce the incidence of many rots in temperate fruits.
Proper pruning can prevent Botrytis rot of
grapes (Philips et al., 1990).
The influence of N, P, K, Ca and Mg
nutrients on storage rots of apple and pear
has been studied extensively (Sharples, 1980).
Susceptibility to Gloeosporium rot was correlated negatively with fruit Ca, but correlated positively with K/Ca ratios. Higher
doses of nitrogen increase the incidence of
G. album (Montgomery and Wilkinson, 1962).
Calcium sprays to control bitter pit in apples
also confer resistance to P. expansum.
Fumigation
Safe fumigating agents that disappear after
a short time, such as the use of ozone and
sulphur dioxide and acetic acid, can be
recommended to reduce dependence on conventional fungicides. Ozone application to
grapes (0.1 mg/g grapes) during 20 min exposure reduced decay caused by Rhizopus
stolonifer and prolonged shelf life. This treatment was as effective as sulphur dioxide
5
(Sarig et al., 1996). The tolerance of grapes
to sulphur dioxide is unique among fresh
fruits. It eradicates most of the postharvest
pathogens. However, the benefits of sulphur
dioxide disappear after a short period of
time. Hence, sodium bisulphate in packing
cases reacts with the moisture in the air in
grape containers. This treatment is used
exclusively for the long distance transportation of grapes (Hedberg, 1977). Fumigation
with acetic acid is effective in controlling
M. fructicola, R. stolonifer and Alternaria
species on peaches, nectarines, apricot and
cherries (Sholberg and Gaunce, 1996). Relatively few fumigation treatments have been
developed for pome and stone fruits.
Heat treatments
Heat treatments may be applied by hot water
dips or hot vapour exposure. Hot water is
useful in controlling fungal infections, while
exposure to hot vapour controls insects.
Postharvest decay of strawberries caused by
B. cinerea and R. stolonifer has been controlled by exposing the fruits to humid air at
44°C for 40–60 min (Couey and Follstad,
1966). Akamine and Arisumi (1953) have
reported hot water treatments for fruit rot of
papaya (48°C for 20 min). Two methods
have been suggested: one involves a shortterm heat treatment above 40°C (usually
44–55°C) for a few minutes to 1 h and in the
other, the fruits are exposed to 38–46°C but
for a longer duration (12 h to 4 days) (Fallik
et al., 1996). The LD50 temperature for sporangiospores of R. stolonifer exposed to hot
water for 4 min was 49°C, whereas that for
germinating spores was only 39°C (Eckert
and Sommer, 1967).
Ionizing radiation and UV illumination
Ionizing radiation may harm the genetic
material of the living cell directly, leading
to mutagenesis and eventually to cell death.
Most studies are carried out with Co60 gamma
rays. It has been seen that multicellular
conidia of Alternaria and Stemphylium or
6
A. Arya
bicellular spores such as Cladosporium and
Diplodia are more resistant to gamma radiation than the unicellular spores of other
fungal species (Sommer et al., 1964). Since
radiation can penetrate fruit tissues, it has a
therapeutic effect. Plant tissues can produce
phytoalexins (defence chemicals) in response
to radiation effect. Low doses of UV-C light
(wavelength 190–280 nm) can induce resistance in a wide range of fruit and vegetables
(Barkai-Golan, 2001). UV light has a germicidal effect and, at the same time, it induces
activity of PAL and peroxidase enzymes
(Droby et al., 1993).
Another treatment for extending the
postharvest life of apple, pear and plum is
by coating the skin with a product called
‘Prolong’, a mixture of sucrose esters of fatty
acids and polysaccharide (Banks and
Harper, 1981). It alters the permeability of
fruits to gases in such a way that oxygen
permeability is reduced considerably, while
carbon dioxide permeability is little affected.
This coating had little effect on grapes and
strawberries.
Chemically impregnated wrappers
Various strains of antagonist must be compared for effectiveness in controlling fruit
decay and for phenotypic characteristics
that are useful in determining their commercial potential; for example, the differentiation criteria for decay control on apple
includes the biological control efficacy of
the strains, spectrum of activity (pathogens
to be tested, cultivar range, fruit maturity
stages), ability to colonize wounded and
sound fruit surfaces under various conditions, utilization of substrates occurring in
fruits, or growth at cold storage temperatures and at 37°C.
In addition, these antagonists must
meet strict regulations for safety as they are
being applied to consumable commodities,
i.e. fruits. Thus, in developing biocontrol
systems for postharvest disease management of fruits, the key requirements for successful commercialization of an antagonist
must be well defined and strain searches
should continue until adequate strains are
found that meet all the safety requirements.
Wrapping grape clusters in tissue paper
impregnated with sodium orthophenyl butyrate and sodium metabisulphate reduces
postharvest decay. Volatile fungal inhibitors also provide effective control of grapes
against A. niger and P. canescens (Sharma
and Vir, 1984). Potassium iodide wraps provide effective control of G. roseum on apples
(Sharma and Kaul, 1988). Development of
Botryodiplodia rot of apples was retarded
by wrapping them in papers dipped in culture filtrate of Streptomyces thermoflavus
(Gupta and Gupta, 1983).
Fruit skin coatings
Skin coatings can improve the keeping quality of fruits by decreasing water loss and
retarding ripening and rotting by various
pathogens. Coating is generally done with
oils, waxes and colloidal solutions of carboxymethyl cellulose. Apples coated with
mustard oil, paraffin and castor oil checked
the infection of a large number of pathogens
(Sumbali and Mehrotra, 1980; Kaul and Munjal, 1982; Sharma and Kaul, 1988). Application of hydrogenated groundnut oil provided
effective control of Alternaria rot of apple
(Tak et al., 1985). Skin coating with neem
oil completely checked blue mould rot in
apples (Kerni et al., 1983).
Search for the antagonists:
criteria of selection
Enhancement in biocontrol
activity of antagonists
Postharvest environments are better defined
than field conditions, wherein abiotic and
biotic factors can be determined with relative ease and manipulated to the antagonist’s advantage, although the mechanism(s)
of biocontrol have not yet been fully explained
Recent Advances in the Management of Fungal Pathogens of Fruit Crops
and, to date, there have been only a few
attempts to exploit these mechanisms to
improve postharvest biocontrol (Janisiewiez et al., 1992). The reports available on the
mechanism of the biocontrol of postharvested commodities suggest that competition for nutrients and space plays a major
role in most cases (Wisniewski et al., 1991;
Calvente et al., 1999). In most of the systems
where microbial communities are involved,
interactions are density dependent and often
more than one type of interaction occurs at a
specific time which is dependent on the
growth phase of different microorganisms,
population density and species diversity.
Basically, three different types of interactions, namely competition for nutrients,
competition for space and inhibition by secondary metabolites, have been observed in
preharvest sprays of B. subtilis to control C.
gloeosporioides on avocado (Korsten et al.,
1997). The main approaches used to improve
biological control in postharvest systems
are: (i) manipulation of the environment; (ii)
use of mixed cultures of antagonists; (iii)
physiological and genetic manipulation of
antagonists; (iv) combining field and postharvest applications; (v) manipulation of
formulations; and (vi) integration with other
methods.
In the case of the development of BioSave, the effectiveness of the antagonist, a
saprophytic strain of P. syringae L-59-66, in
reducing blue mould and grey mould decay
on apples and pears in a laboratory setting
was demonstrated to EcoScience Corp
(Orlando, Florida, USA). The commercial
setting of the test, the involvement of industry in conducting those tests and the encouraging results were the key factors in obtaining
a commitment to develop the antagonist for
commercial use. EcoScience Corp then investigated the potential for registration and formulation of the antagonist before making
this commitment. Mass production by fermentation and the biomass yield of P. syringae strain L-59-66 was determined before
scale-up experiments (Janisiewiez, 1998).
Extensive technical support and quality control have been instrumental in the success of
this product. Similar support and testing
need to be conducted for the development
7
of many more biocontrol agents for postharvest fruit rots.
Biocontrol: an integrated approach
Recently there has been an increased interest in enhancing the efficacy of biocontrol
agents by adding some synthetic chemicals
like calcium chloride or nitrogenous compounds or sugar analogues. For example, a
mixture of Cryptococcus laurentis and thiabendazole has been observed to reduce
95% of P. expansum infection in pear (Sugar
et al., 1994). Enhancement of biocontrol activity of antagonists by the addition of nitrogenous (L-asparagine, and L-proline) and
carbohydrate (2-deoxy-D-glucose) compound
has been reported in apple and pear fruit
(Janisiewiez, 1994). Similarly, a combination of 2-deoxy-D-glucose and Candida
saitoana is reported to be useful in reducing
postharvest diseases (Wilson and El-Ghaouth,
1997). Recently, a bioactive coating having a
combination of C. saitoana and 0.2% glycolchitosan has been found more effective in
controlling rot development caused by B.
cinerea, P. digitatum and P. expansum in several cultivars of apples, oranges and lemon
(El-Ghaouth et al., 2000a,b). The same group
of researchers showed that the application
of C. saitoana with 0.2% 2-deoxy-D-glucose,
before inoculation of pathogens, was more
effective in controlling the decay of apple,
orange and lemon caused by B. cinerea, P.
expansum and P. digitatum than either C.
saitoana or 0.2% 2-deoxy-D-glucose alone.
For the postharvest treatment of fruits,
stock of biocontrol agent is usually made in
lyophilized cultures, agar slant or spore suspensions and is maintained at low temperature and at the same osmotic concentration
in culture medium (Churchill, 1982).
Botanicals as Antifungal Agents in
Postharvest Disease Control of Fruits
Fruits and vegetables have a number of constituents and inducible volatile aromatic
and flavour compounds (Tripathi, 2007).
8
A. Arya
These aromatic and flavour components are
produced generally by fruits during ripening
and provide resistance to the fruits at the
postharvest stage. The flavour compounds
are secondary metabolites having unique
properties of volatility and low water solubility. As potential fungicides, their natural
occurrence as part of the diet, their ephemeral
nature and their biodegradability suggest low
toxic residue problems. Such compounds
could be extracted and applied to other
harvested perishables. Some of the volatile
aromatic components, namely acetaldehyde, six carbon (C6) aldehydes, benzaldehyde, hexenel and hexanal, are of significant
importance.
Vapours of acetaldehyde have been
used to control B. cinerea (Prasad and Stadelbacher, 1973). Avissar and Pesis (1991)
reported acetaldehyde to be active against B.
cinerea and R. stolonifer causing rot to strawberry fruits. The effect of trans-2-hexenel
on the control of blue mould disease (P.
expansum) in the reduction of patulin content and on fruit quality improvement of
‘Conference’ pears was evaluated and
greater reduction of decay was obtained by
treatment at 12.5 µl/l at 20°C for 24 or 48 h
after inoculation (Neri et al., 2006).
Jasmonates are naturally occurring
plant growth regulators that are widely distributed in the plant kingdom and are
known to regulate various aspects of plant
development and responses to environmental stresses. The antifungal activity of six
glucosinolates has been tested on several
postharvest pathogens, namely B. cinerea,
R. stolonifer, Monilinia laxa, Mucor piriformis and P. expansum, both in vitro and
in vivo (Mari et al., 1996).
Fumigation of apples with acetaldehyde,
a natural volatile compound produced by
various plant organs, inhibits P. expansum
development in the fruit (Stadelbacher and
Prasad, 1974), while fumigation of strawberries with acetaldehyde considerably reduces
decay caused by R. stolonifer and B. cinerea.
Evaluation of 15 volatile odour compounds,
released from raspberries and strawberries
during ripening, for their ability to inhibit
postharvest decay fungi showed that 5 of
them inhibited the growth of A. alternata,
B. cinerea and C. gloeosporioides directly on
the fruit at 0.4 µl ml (Vaughn et al., 1993).
Among the five compounds, benzaldehyde
was the most toxic to the fungi.
Plant extracts
Fungitoxic activity of plant extracts can be
tested by the poisoned food technique (Grover and Moore, 1962). Tripathi (2005) tested
24 taxa belonging to 12 different families for
their antifungal activity against P. italicum.
Most of the plants showed either poor or
moderate (50–100%) activity. Leaf extracts of
seven plants, namely Acacia nilotica (ethyl
alcohol), Citrus aurantifolia (ethyl acetate),
Murraya koenigii (ethyl acetate), Nerium
indicum (ethyl acetate), Ocimum gratissimum (benzene, ethyl acetate), O. sanctum
(petroleum ether), Prunus persica (ethyl acetate) and bark extract of A. farnesiana and A.
nilotica (ethyl acetate extract) showed 100%
activity against test fungus. The leaves of
Achyranthes aspera and Hyptia suaveolens
showed poor activity.
Arya (1988) tried leaf extracts of Aegle
marmelos, O. sanctum, Azadirachta indica,
Crataeva nurvala, Ephedra foliata (shoot),
Eucalyptus occidentalis, Lawsonia inermis
and Strichnos nux vomica in three different
concentrations on two fruit rot pathogens,
P. psidii and P. viticola. Extracts obtained
from Ephedra and Eucalyptus were most
effective at 25% concentration in the case of
P. viticola, while a higher concentration
(75%) leaf extract of ‘neem’ (A. indica) was
most effective, causing 82.3% spore inhibition. Tulsi caused 76.4% inhibition. The
fungicidal nature of ‘neem’ and ‘tulsi’ was
reported earlier by Pandey et al. (1983)
against Pestalotia psidii.
Essential oils
Volatile oils are sweet-smelling lipids synthesized and stored in various plant parts. These
oils are essentially mixtures of two classes
of terpenoids, i.e. the monoterpenes and the
sesquiterpenes, the former predominating in
Recent Advances in the Management of Fungal Pathogens of Fruit Crops
most cases. Among the 49 essential oils tested,
those of palmrosa (Cymbopogon martini) and
red thyme (Thymus zygis) showed the greatest inhibitory effect on B. cinerea spore germination at the lowest concentration. The
next best inhibitors were essential oils of
clove buds (Eugenia caryophyllata) and cinnamon leaf (Cinnamomum zeylanicum). The
most frequently occurring constituents in
essential oils showing high antifungal activity
were: D-limonene, cineole, a-pinene, b-pinene,
b-myrcene and camphor. The fungicidal
activity of the individual components, singly and in combination, is being studied
(Wilson et al., 1997). Essential oil derived
from another species of Thymus, T. capitatus, reduced the development of B. cinerea
markedly in inoculated mandarin fruits when
applied as a vapour. Scanning electron microscopic observations indicated a direct damaging effect of the thyme oil on fungal
hyphae (Arras and Piga, 1994).
Gel and latex
Gel derived from Aloe vera has been found
to have antifungal activity against four common postharvest pathogens, P. digitatum, P.
expansum, B. cinerea and A. alternata. The
natural gel suppressed both germination
and mycelial growth. Latex present in some
fruits is another natural fungicide which is
effective against diseases of banana, papaya
and other fruits (Adikaram et al., 1996).
Papaya latex contains proteases, glucosidases, chitinases and lipases, while a cysteinrich protein, hevien, was isolated from the
latex of rubber tree (Hevea brasiliensis). It
showed a strong antifungal activity in vitro
against B. cinerea and species of Fusarium
and Trichoderma (van Parijs et al., 1991).
Use of Plantibodies for
Disease Control
Drawing a clue from the potential antibodies
in combating human diseases, plant scientists are now geared to extend this remarkable technology to plant disease control.
9
Antibodies are produced in response to invasion of an antigen. The remarkable potential
of recombinant DNA technology has made
it possible for plants to express antibodies
against pathogen proteins, which in turn
enable them to defend against the target
pathogen. The expression of pathogenspecific antibody in plants is termed ‘plantibody’ (Smith, 1996; Gibbs, 1997). The
plantibodies produced in the cell cytosol are
expected to interact with their targets, rendering them inactive (Zhang and Wu, 1998).
Induced Resistance
Induced resistance is a new concept proposed
by the American phytopathologist, Joseph
Kuc (1995). According to Kuc, resistance in
plant tissues can be enhanced by modulating their natural defence mechanisms. Various physical, chemical and biological elicitors
can enhance resistance in plants. Use of chitosan, a deacetylated derivative of chitin,
and salicylic acid can be made to offer a
possible alternative to synthetic pesticides.
ASM (acibenzolar-s-methyle) is the first
commercially available product that activates a systemic acquired resistance (SAR)
in plants like other biological inducers.
Host Defence Through
Gene Silencing
Scientists working on Eutypa dieback disease of grapevine in Switzerland (2008)
found the involvement of glutathion-stransferase in the detoxification of toxins, of
the jasmonic acid signalling path way, and
of several effector genes underlying a more
general response where the toxins could be
recognized as an elicitor for the trunk pathogens. Grapevines were tested for infiltration
of double standard RNA into leaves for easy
testing of genes. dsRNA were functional in
Puccinia striiformis to suppress recognition
by host plants (Newton, 2002). Genes that
encode for post-transcriptional gene silencing have been characterized in plants and
fungi (Dalmay et al., 2000).
10
A. Arya
A variety of gene silencing phenomena
that have been discovered are: (i) the duplicated DNA sequence is inactivated by mutation in the meiotic phase, a process known
as repeat induced mutation (RIP) (Selker
et al., 1987); (ii) the duplicated DNA
sequence during the meiotic phase is inactivated by methylation, methylation induced
premeiotically (MIP) (Goyon and Faugeron,
1989); (iii) multiple copies of transgenes in
the vegetative phase are irreversely inactivated and silencing is called ‘quelling’
(Romano and Macino, 1992); and (iv) silencing is maintained even in the absence of
transgenes (van West et al., 1999) or another
process called MSUD (Shiu et al., 2001).
Disease-resistant Transgenic Plants
Newly developed techniques in plant breeding such as restriction fragment length polymorphism techniques and gene transfer
methods can be used to develop these cultivars. In contrast to conventional breeding, this later technology allows the transfer
of traits from one species into the genomes
of plants of other species with the preservation of the intrinsic properties of the
acceptor plant (Cornelissen and Melchers,
1993).
A transgenic plant contains, within its
genome, a foreign DNA that has been introduced artificially via genetic engineering. The
creation of such plants involves the introduction of genes for resistance from unrelated plant species. Desirable target genes
are isolated from plant viruses, bacteria,
fungi or other plants and introduced in the
plants. Genes have been transferred by scientists in India from Amaranthus to potato
for improving protein quality and quantity,
and from mangroves to annual crops for
imparting tolerance to salinity. Powell et al.
(1994) reported that transgenic tomato fruits
expressing the gene of fungal PG-inhibiting
glycoproteins of plants were more resistant
to B. cinerea than the control fruits. Scientists have tried to prevent ethylene production by plant tissue using an antisense gene.
The fruits would not ripen here until treated
exogenously with ethylene. PR protein genes
appear to be a very potential source for candidate genes providing fungal resistance.
These proteins may play a direct role in
defence by attacking and degrading pathogen cell wall components.
The first specific fungal-resistant gene,
Hm1, has been isolated from maize, conferring resistance to race 1 of the fungus Helminthosporium carbonum (Johal and Briggs,
1992). After fungal-resistance genes have
been isolated, they can be transferred to provide resistance to a specific race of fungal
pathogens. Woloshuk et al. (1991) identified
in tobacco a salt stress-inducible vacuolar
protein with an inhibitory effect on the
growth of P. infestans in vitro. It was suggested that this protein, described as Osmotin, inhibited growth by interfering with the
fungal membrane, hence disturbing cellular
function. As with class I hydrolyses, the
protein could be arrested extracellularly by
modification of the corresponding gene
(Melchers et al., 1993).
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2
Botanicals in Agricultural
Pest Management
Ashok Kumar, Priyanka Singh and N.K. Dubey
Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi, India
Abstract
The overzealous and indiscriminate use of most of the synthetic fungicides has created different types
of environmental and toxicological problems. The ultimate aim of recent research in this area has been
the development of alternative control strategies to reduce dependency on synthetic fungicides.
Recently, in different parts of the world, attention has been paid to the exploitation of higher plant
products as novel chemotherapeutants in plant protection because of their non-phytotoxicity, systemicity and easy biodegradability. The exploitation of natural products to control fungal infestation and
prolong storage life of food commodities has received more attention. Biologically active natural products have the potential to replace synthetic fungicides. Currently, different plant products have been
formulated for large-scale application as botanical pesticides in the eco-friendly management of plant
pests and are being used as alternatives to synthetic pesticides in crop protection. This chapter deals
with the current status and future prospects of botanical pesticides in eco-friendly management of different plant pests.
Introduction
The constant growth of the world’s population requires substantial resources for the
production of food. One of the greatest challenges of the world is to produce enough
food for the growing population. Production as well as protection of food commodities is necessary to nourish the ever-growing
population. The situation is particularly
critical in developing countries, where the
rate of net food production is slowing down
in relation to population growth. The world
food situation is aggravated by the fact that
in spite of the use of all available means of
plant protection, a major proportion of the
yearly production of food commodities of
14
the world is destroyed by various pests,
including bacteria, fungi, viruses, insects,
rodents, nematodes, etc. Losses at times are
so severe as to lead to famine in large areas
of the world that are densely populated.
Considerable attention has been given to
losses in the field caused by different pests,
but research into postharvest losses of food
commodities is still required. So, priority
should be given to postharvest studies, particularly in hot and humid tropical climates
where at least half of the foodstuffs may be
lost between harvest and consumption. Considerable postharvest losses of food commodities are brought about due to fungi, insects
and rodents. International agencies that monitor world food resources have acknowledged
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Botanicals in Agricultural Pest Management
that one of the most feasible options for meeting future food needs is the reduction of postharvest losses (Tripathi and Dubey, 2004).
Fungi are significant destroyers of foodstuffs during storage, rendering them unfit
for human consumption by retarding their
nutritive value. Many agricultural commodities are vulnerable to attack by a group of
fungi that are able to produce toxic metabolites called mycotoxins. Production of mycotoxins by several fungi has added a new
dimension to the gravity of the problem.
Fungal toxins are low molecular weight
chemical compounds which are not detected
by the body’s antigens. Their effect is more
often chronic rather than acute; hence, they
produce no obvious symptoms. Thus, mycotoxins are insidious poisons (Pitt, 2002).
Cereals and grains are major mycotoxin vectors because they are consumed by both
humans and animals. According to FAO
estimates, 25% of the world food crops are
affected by mycotoxins each year. These toxins can develop during production, harvesting, or storage of grains, nuts and other crops.
Mycotoxins are among the most potent mutagenic and carcinogenic substances known.
They pose chronic health risks: prolonged
exposure through diet has been linked to
cancer and kidney, liver and immune system disease (Srivastava et al., 2008). Among
mycotoxins, aflatoxins chiefly produced by
strains of Aspergillus flavus are the most
dangerous and about 4.5 billion people in
underdeveloped countries are at risk of
chronic exposure to aflatoxicosis through
contaminated foods (Williams et al., 2004;
Srivastava et al., 2008). In most of the developing countries, total permissible aflatoxin
content in food has been set around 20 ppb
(Mishra and Das, 2003). Aflatoxins are potent
toxic, carcinogenic, mutagenic, immunosuppressive agents, produced as secondary
metabolites by the fungus Aspergillus, A.
parasiticus and A. nomius on a variety of
food products. In addition, aflatoxin inhibits seed germination, seedling growth, root
elongation, chlorophyll and carotenoid synthesis, as well as protein, nucleic acid and
some enzyme synthesis in seeds.
Climatic conditions in India are most
conducive to mould invasion and elaboration
15
of mycotoxins. Unseasonal rains and flash
floods are very common in India, which
enhances the moisture content of the grains,
making them more vulnerable to fungal attack
(Srivastava, 1987). Fungi can grow on simple
and complex food products and produce various metabolites (Khosravi et al., 2007). Up to
now, more than 100,000 fungal species are
considered as natural contaminants of agricultural and food products (Kacaniova, 2003).
The quality and safety of food is of importance
so that markets are not compromised by the
sale of low quality or unsafe food.
Control of Fungal Infestation
During Storage
Attempts to control postharvest diseases
have been carried out by different physical
and chemical treatments.
Physical methods
Several techniques are used for the preservation of food and feeds. Drying, freezedrying, cold storage, modified atmosphere
storage and heat treatments are all physical
methods of food preservation (Farkas, 2001)
(Table 2.1).
Cold storage
Low temperature inhibits the germination of
spore/conidia and pathogenicity significantly
(Tian, 2001). It reduces the metabolic activities of various microbes associated with foodstuffs, which would be helpful in enhancing
the shelf life of edibles. However, cold storage
has its limitations, such as unavailability in
most developing countries and an inability to
check psychrophilic microorganisms.
Heat treatment
High temperature plays a significant role
in controlling the metabolic activities of
organisms because it affects the enzymatic
activities in all organisms adversely (Lagunas and Castaigne, 2008; Moatsou et al.,
2008). Heat treatment can check microbial
16
A. Kumar et al.
Table 2.1. Some physical and chemical methods used in the prevention of fungal contamination and
mycotoxin production.
Methods
Fungi/mycotoxins
References
Physical:
Sunlight
Solar irradiation
Electric light
UV light
UV-C radiation
Infrared light
γ Radiation
α Radiation
Autoclaving
Cooking
Roasting
Dry heat
Low temperature/
refrigeration
Aspergillus flavus/aflatoxin
A. parasiticus/aflatoxin B1
A. flavus/aflatoxin B1
A. flavus/aflatoxin
Colletotrichum gloeosporioides
Penicillium citrinum
Cryptococcus neoformans
C. neoformans
All types of moulds
Food-spoiling moulds
Food-spoiling moulds
Fusarium graminearum
Some soil fungi
Shantha and Sreenivasamurthy (1977)
Samarajeewa et al. (1985)
Chourasia and Roy (1991)
Shantha and Sreenivasamurthy (1977)
Cia et al. (2007)
Qing et al. (2002)
Dadachova et al. (2004)
Martinez et al. (2006)
Coomes et al. (1966)
Rehana and Basappa (1990)
Ogunsanwo et al. (2004)
Clear et al. (2002)
Janna et al. (2005)
A. flavus/aflatoxin
A. flavus/aflatoxin
C. lupini
C. lupini
C. lupini
A. carbonarius/ochratoxin
Penicillium sp., Trichoderma sp.
F. graminearum/ZEN
Sclerophoma pityophila
C. gloeosporioides
Aspergillus spp.
Aspergillus spp.
Aspergillus spp.
Aspergillus spp.
A. flavus
A. flavus
A. flavus
A. flavus
A. flavus
Sreenivasamurthy et al. (1967)
Shantha et al. (1986)
Thomas et al. (2008)
Thomas et al. (2008)
Thomas et al. (2008)
Medina et al. (2007)
Magarey et al. (1997)
D’Mello et al. (1998)
Olender et al. (2008)
Rehman et al. (2008)
Satish et al. (2008)
Satish et al. (2008)
Satish et al. (2008)
Satish et al. (2008)
Kumar et al. (2008)
Kumar et al. (2008)
Kumar et al. (2008)
Kumar et al. (2008)
Kumar et al. (2008)
Chemical:
H2O2
Na-hypochlorite
Azoxystrobin
Chlorothalonil
Copper oxychloride
Carbendazim
Mancozeb
Maneb
Nitroimidazole
Organotin
Blitox
Captan
Dithane M-45
Thiram
SAAF
Bavistin
Wettasul-80
Ceresan
Diphenylamine
growth efficiently but the technique is not
suitable for long-term storage.
Radiation
Sun drying of food commodities (grains and
pulses) before storage is preferable in most
underdeveloped countries but the technique is unsuitable in the case of vegetable
crops. High-energy radiation like γ rays
(Petushkova et al., 1988), UV rays (Oteiza
et al., 2005), infrared (Qing et al., 2002), etc.,
is also efficient in checking microbial growth
and proliferation, as well as mycotoxin
production. The irradiation of food commodities during storage is unattainable in
developing countries.
Chemical methods
In order to minimize the losses caused by
moulds in the field and also during storage,
Botanicals in Agricultural Pest Management
many synthetic fungicides have been introduced (Table 2.1). The discovery of Bordeaux mixture is significant in the history of
the chemical control of plant diseases. In the
past few decades, various synthetic chemicals have played a significant role in the
management of such losses. Several chemical additives also function as preservatives,
even though the exact mechanisms or targets are often not known (Davidson, 2001.
The organic acids, acetic, lactic, propionic,
sorbic and benzoic acids, are used as food
preservatives (Brul and Coote, 1999). Both
sorbic and benzoic acid have a broad spectrum of activity (Nielsen and De Boer, 2000;
Davidson, 2001). Benzoic acid and sodium
benzoate are used primarily as antifungal
agents (Davidson, 2001). Recently, some
technology like TiO2 photocatalytic ozonation has been found to be efficient in controlling postharvest spoilage of kiwifruit
(Hur et al., 2005).
The indiscriminate application of synthetic chemicals as antimicrobials has contributed greatly to the management of losses
caused by fungi, but these chemicals have
led to a number of ecological and health
problems due to their residual toxicity
(Kneževi and Serdar, 2008), carcinogenicity,
teratogenicity, hormonal imbalance, spermatotoxicity, etc. (Pandey, 2003; Kumar
et al., 2007). History also shows that overzealous use of synthetic pesticides has led
to numerous problems unforeseen at the
time of their introduction. Different types of
ecological problems have been reported
from time to time by these xenobiotics, such
as acute and chronic poisoning of applicators, farm workers, and even consumers,
extensive groundwater contamination, resistance development in pests (Wilson et al.,
1997), effect on non-target organisms (Wiktelius et al., 1999), ozone layer depletion by
methyl bromide (Lee et al., 2001), etc.
Biocontrol Agents in
Pest Management
Considerable attention has also been given
to the potential of biological control of
17
postharvest diseases of fruits, vegetables and
other edibles as a viable alternative to the use
of present day synthetic fungicides (Wilson
et al., 1999; Pang et al., 2002). Microbial
antagonists have been reported to protect a
variety of harvested perishable commodities against a number of postharvest pathogens (Wisniewski et al., 2001). However,
decreasing efficacy and lack of consistency
when applied as stand-alone treatments
under commercial conditions (Droby et al.,
2001) are limiting their use. Hence, these
drawbacks in alternative methods have
increased interest in developing further
alternative control methods, particularly
those which are environmentally sound and
biodegradable.
Botanicals as Fungitoxicants
Recently, in different parts of the world, attention has been drawn towards the exploitation
of higher plant products as novel chemotherapeutants in plant protection. Because of
non-phytotoxicity, systemicity, easy biodegradability and the stimulatory nature of
host metabolism, plant products possess the
potential to be of value in pest management
(Mishra and Dubey, 1994). Higher plants contain a wide spectrum of secondary metabolites such as phenols, flavonoids, quinones,
tannins, essential oils, alkaloids, saponins
and sterols. Such plant-derived chemicals
may be exploited for their different biological properties (Tripathi et al., 2004). Terrestrial plants produce a spectrum of natural
products, namely terpenoids, phenolics and
alkaloids. Many of these are thought to have
an ecological function for the plants producing them, serving to defend the plants
from herbivores and pathogens (Isman and
Akhtar, 2007). Such defensive chemistry is
thought to be extremely widespread among
the plant kingdom.
The body of scientific literature documenting the bioactivity of plant derivatives
to different pests continues to expand; yet
only a handful of botanicals are currently
used in agriculture in the industrialized
world. In the context of agricultural pest
18
A. Kumar et al.
management, botanical pesticides are well
suited for use in industrialized countries
and can play a much greater role in the postharvest protection of food commodities in
developing countries (Isman, 2006).
Among the different plant products, the
application of essential oils is a very attractive method for controlling postharvest losses
(Table 2.2). Production of essential oils by
plants is believed to be predominantly a
defence mechanism against pathogens and
pests (Oxenham, 2003). Essential oils and their
components are gaining increasing interest
because of their relatively safe status, wide
acceptance by consumers and their exploitation for potential multi-purpose use (Sawamura, 2000; Ormancey et al., 2001; Feng and
Zheng, 2007). The problem of the development of resistant strains of fungi and other
organisms may be solved by the use of
essential oils of higher plants as fumigants
in the management of storage pests because
of synergism between different components
of the oils (Varma and Dubey, 1999; Dubey
et al., 2006).
The antifungal activity of essential oils
is well documented and characterized with
their bioactivity in vapour phase. The pesticidal activities of essential oils are due to
the presence of some aroma compounds.
Fumigation with such aroma compounds
greatly reduces postharvest decay without
causing any toxicity (Chu et al., 2001; Liu
et al., 2002). Recently, some monoterpenes
isolated from essential oils exhibited fungicidal activity and have been shown to
inhibit fungal rotting of vegetables without
altering taste and quality (Hartmans et al.,
1995; Oosterhaven, 1995). The fungitoxic
properties of essential oils from higher
plants are well documented but little attention has been paid towards the bioactivity
of essential oil constituents. The fungitoxic
activity of some essential oil components
is listed in Table 2.3 and Fig. 2.1. However,
more work on the bioactivity of plant products including essential oil and constituents in in vitro and in vivo conditions is
required. The literature is also silent on the
mode of action of the essential oils and
components when used as postharvest
fungitoxicants.
Conclusions
Plants are a virtually untapped reservoir of
different valuable chemicals that can be
used directly or as templates for the formulation of pesticides. Numerous factors
have increased the interest of the pesticide
industry and the pesticide market in this
source of natural products as pesticides.
Pesticides based on plant essential oils or
their constituents have demonstrated their
efficacy against a range of fungal pests
responsible for pre- and postharvest diseases,
as well as mycotoxin production. Encouraging results on the use of natural products to
control postharvest fungal spoilage indicate
that we should be able to develop natural
pesticides that could be as effective as synthetic fungicides and presumably safer for
man and the environment. Biological compounds, because of their natural origin, are
comparatively biodegradable and most of
them are almost non-residual in nature (Beye,
1978).
During recent years, products of some
pesticidal plants have received global attention for the protection of several food commodities because of their antimicrobial
properties (Kumar et al., 2007). Such plant
products have been formulated for largescale application as botanical pesticides,
which are used as alternatives to synthetic
pesticides in crop protection. A consolidated and continuous search of natural
products may yield safer alternative control
measures comparable to azadirachtin and
pyrethroids, which are being used in different parts of the world as ideal natural fungicides. The number of options that must be
considered in the discovery and development of a natural product as a pesticide is
larger than for a synthetic pesticide. However, current advances in plant chemistry
and biotechnology, combined with increasing need and environmental pressure, are
greatly increasing the interest in plant products as pesticides. Products from higher
plants are a safe and economical option in
the management of agricultural pests and
will be in high demand in the global pesticide market.
Botanicals in Agricultural Pest Management
Table 2.2.
19
Efficacy of some higher plant products in checking fungal growth and mycotoxin production.
Plants
Products
Hypericum linarioides
EO/PEE/
6 Fusarium spp.
ME/ChlE
EO
Colletotrichum gloeosporioides,
Rhizoctonia solani, F. oxysporum
EO/ME/
F. oxysporum, C. capsici,
HexE
Botrytis cinerea
EO
17 pathogenic fungi
EO
Trametes versicolor, Lenzites
betulina, Laetiporus sulphureus
EO
Candida albicans
EO
Aspergillus niger,
A. parasiticus
EO
F. oxysporum, Cladosporium
herbarum, A. flavus
EO
C. albicans
Cakir et al. (2005)
AqE/EO
Omidbeygi et al. (2007);
Aldred et al. (2008);
Reddy et al. (2008)
Reddy et al. (2008)
Reddy et al. (2008)
Tatsadjieu et al. (2009)
Viuda-Martos et al.
(2008)
Deba et al. (2008)
Omidbeygi et al. (2007);
Abyaneh et al. (2008);
Dikbas et al. (2008)
Rasooli et al. (2006)
Rasooli and Abyaneh (2004)
Atanda et al. (2007)
Matan and Matan (2008)
Pinto et al. (2007)
Calocedrus macrolepis
Silene armeria
Origanum acutidens
Cinnamomum
osmophloeum
Thymus numidicus
Lantana camara
O. glandulosum
Tarchonanthus
camphoratus
Syzygium aromaticum
Curcuma longa
Allium sativum
Lippia rugosa
Citrus sp.
AqE
AqE
EO
EO
Bidens pilosa
Satureja hortensis
EO/AqE
EO/ME
T. eriocalyx
T. x-porlock
Ocimum basilicum
Pimpinella anisum
Salvia officinalis
EO
EO
EO
EO
EO
T. vulgaris
Cympopogon citratus
EO
EO
Fungi/mycotoxins
A. flavus/aflatoxin B1,
A. flavus, Penicillium
verrucosum/ochratoxin A
A. flavus/aflatoxin B1
A. flavus/aflatoxin B1
A. flavus/aflatoxin B1
A. flavus, P. chrysogenum,
P. verrucosum
Corticium rolfsii, F. solani
A. flavus, A. parasiticus/aflatoxin
A. niger
A. parasiticus/aflatoxin
A. parasiticus/aflatoxin
A. niger, P. chrysogenum
C. albicans, Trichophyton
rubrum, A. flavus
A. flavus/aflatoxin B1
B. cinerea, C. herbarum,
A. niger
A. parasiticus/aflatoxin
A. parasiticus/aflatoxin
R. solani, T. mentagrophytes
Rosmarinus officinalis
EO
Trachyspermum copticum EO
Cordia curassavica
EO/HexE/
ChlE/ME
Sesuvium portulacastrum EO
A. niger, A. flavus, P. notatum
Calamintha officinalis
EO
B. cinerea
Olea europaea
AE/ME
Alternaria alternata,
A. flavus, F. oxysporum
Citrus sinensis
EO
A. niger
Azadirachta indica
AqE
P. citrinum/Citrinin
Agave asperrima
ME/AqE
A. flavus, A. parasiticus/
aflatoxin B1
Adenocalymma alliaceum AqE
A. flavus/aflatoxin B1
Lupinus albus
AqE
A. flavus/aflatoxin B1
References
Chang et al. (2008)
Bajpai et al. (2008)
Kordali et al. (2008)
Cheng et al. (2006)
Giordani et al. (2008)
Deena and Thoppil (2000)
Bendahou et al. (2008)
Matasyoh et al. (2007)
Kumar et al. (2008)
Tzortzakis and Economakis
(2007)
Rasooli et al. (2008)
Rasooli et al. (2008)
Hernandez et al. (2007)
Magwa et al. (2006)
Bouchra et al. (2003)
Korukluoglu et al. (2008)
Sharma and Tripathi (2008)
Aparecida et al. (2008)
Sánchez et al. (2005)
Shukla et al. (2008)
Mahmoud (1999)
Note: EO, essential oil; ME, methanolic extract; AqE, aqueous extract; PEE, petroleum ether extract; AE, acetone
extract; ChlE, chloroformic extract; HexE, hexane extract.
20
A. Kumar et al.
Table 2.3. Efficacy of some essential oil components in checking fungal growth.
Compounds of
plant origin
Ajoene
Allicin
Myrcene
Limonene
r-Cymene
a-Pinene
Fungi
References
Aspergillus niger, Candida albicans,
Saccharomyces cerevisiae
C. albicans
Rhizoctonia solani, Fusarium oxysporum
Colletotrichum gloeosporioides,
Botryosphaeria parva, F. verticillioides
Fusarium sp.
C. albicans, S. cerevisiae, A. niger
Yoshida et al. (1987)
Naganawa et al. (1996)
Ankri and Mirelman (1999)
Chang et al. (2008)
Regnier et al. (2008)
Dambolena et al. (2008)
Kordali et al. (2008)
Yousefzadi et al. (2008)
Sonboli et al. (2006)
Chang et al. (2008)
Moleyar and Narasimham
(1986)
Da Silva et al. (2008)
Cheng et al. (2006)
Pitarokili et al. (2003)
Regnier et al. (2008)
Duru et al. (2004)
Dambolena et al. (2008)
Shafi et al. (2004)
Zhang et al. (2006)
Zhang et al. (2006)
Moleyar and Narasimham
(1986)
Barra et al. (2007)
Dambolena et al. (2008)
Moleyar and Narasimham
(1986)
Serrano et al. (2005)
Braga et al. (2008)
Kordali et al. (2008)
Dambolena et al. (2008)
Cheng et al. (2006)
Gayoso et al. (2005)
Shafi et al. (2004)
Regnier et al. (2008)
Lee (2007)
Lee et al. (2004)
Agarwal et al. (2001)
Agarwal et al. (2001)
Meepagala et al. (2003)
Meepagala et al. (2003)
Kordali et al. (2008)
Romero et al. (2007)
Chang et al. (2008)
Chang et al. (2008)
Caryophyllene
Citral
R. solani, F. oxysporum
A. niger, F. oxysporum,
Penicillium digitatum, C. albicans
Cinnamaldehyde
Camphor
Carvone
Pulegone
Menthone
Thujone
Linalool
Geraniol
Citronellol
Lenzites betulina, Laetiporus sulphureus
Fusarium sp., R. solani
C. gloeosporioides, B. parva
C. albicans
F. verticillioides
Phytophthora capsici
C. camelliae
C. camelliae
Rhizopus stolonifer
Terpine-4-ol
Menthol
A. flavus, R. solani, P. commune, F. oxysporum
F. verticillioides, R. stolonifer,
Penicillium sp., Monilia sp.
Thymol
C. albicans, Fusarium sp., F. verticillioides
Eugenol
Zingiberene
Curcumene
Verbenone
Verbenol
Carvacrol
L. betulina, L. sulphureus,
T. mentagrophytes, C. albicans
P. capsici
C. gloeosporioides, B. parva
R. solani, P. infestans, Cladosporium
cucumerinum, Pythium ultimum
R. solani
R. solani
Colletotrichum sp.
Colletotrichum sp.
Fusarium sp., Botrytis cinerea
a-Cadinol
T-muurolol
R. solani, F. oxysporum
R. solani, F. oxysporum
Fenchone
1,8 Cineole
Asarone
Botanicals in Agricultural Pest Management
21
CH3
CH2
CH3
CH3
H3C
CH2
H3C
CH3
CH3
CH2
Myrcene
CH2
H3C
Caryophyllene
H3C
Limonene
CH3
P-Cymene
CH3
OH
CH3
CH3
CH3
H
CHO
O
CH2
H3C
CH2OH
H3C
CH3
Linalool
Camphor
CH3
H
H3C
CH3
Citral
Citronellol
CH3
CH3
H
O
CH3
O
H
O
H3C
O
CH3
H3C
Pulegone
Cinnamaldehyde
H2C
Menthone
CH3
CH3
CH3
CH3
Carvone
CH3
CH3
O
O
CH3
OH
H3C
α-Pinene
CH3
CH3
Terpine-4-ol
H3C
Fenchone
CH3
Thujone
CH2
CH3
CH3
CH2OH
H
OH
OCH3
OH
Eugenol
Fig. 2.1.
H3C
CH3
H3C
Geraniol
CH3
Thymol
Chemical structures of some bioactive essential oil constituents.
continued
22
A. Kumar et al.
CH3
H3C
H3C
CH3
CH3
H
H
CH3
CH3
OH
H3C
H
OH
H3C
CH3
α-Cadinol
Menthol
H
OH
H3C
T-muurolol
CH2
CH3
S
S
CH2
S
H2C
Ajoene
O
O
CH2
S
S
H2C
H3C
Allicin
CH3
1, 8-Cineole
OCH3
CH3
OCH3
CH3
OH
H3CO
CH3
CH3
H3C
CH3
H3C
Zingiberene
Asarone
H3C
CH3
CH3
Carvacrol
H3C
CH3
CH3
CH3
CH3
CH3
H3C
CH3
Curcumene
O
Verbenone
OH
Verbenol
Fig. 2.1. continued.
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3
Deleterious Effects of Fungi on
Postharvest Crops and Their
Management Strategies
A.O. Ogaraku
Plant Science and Biotechnology Unit, Department of Biological Sciences,
Nasarawa State University, Keffi, Nigeria
Abstract
Fungi influence our lives in many ways. The parasitic forms cause serious diseases in crop plants and
pose hazards to the lives of animals and humans whenever they infect consumable crops. Most consumable crops are susceptible to fungal infection. The most prominent types of fungi attacking commodities are species of Aspergillus, Penicillium and Rhizopus, etc. Types of crop deterioration caused
by fungi include discoloration, flavours and odour, rotting and caking, destruction of viability and
production of mycotoxins on food before infestation. Conditions that favour the development of fungi
on harvested and stored crops include moisture, preharvest infection and lapses in the processing
method. Method of control involves drying of produce to a safe moisture level, non-mixing of new
produce with old ones, avoidance of pre-storage damage and use of chemicals, fungicides and medicinal plants in treating the produce.
Introduction
Fungi are one of the most important groups
of organisms on the planet. They are microscopic, achlorophyllous and non-vascular
plants. They cause deterioration of postharvest crops (Ogundana et al., 1970).
Deterioration means that something is
made to be of less value or worse in quality
(Adebayo et al., 1994). It is a common phenomenon in agricultural crops, either on the
farm, at harvest or during storage. Fungi are
known to cause various types of deterioration and pose a hazard to humans and animals whenever they infect crops. Fungal
deterioration can be defined as any change
resulting from the activities of fungi which
renders a product unsuitable for its intended
28
use or reduces the economic value of the
materials (Opadokun et al., 1979).
It is also noteworthy to mention some
other factors that have been identified as
causing damage to crops, namely:
●
●
●
●
●
insects and mites
microorganisms, such as bacteria, actinomycetes, yeasts and virus
rodents and birds
physical factors, such as temperature
and relative humidity of the storage
environment
harvesting, handling and transportation
(Clarke, 1968).
Before the 17th century, scientists concentrated on damage caused by insects on stored
products. This was because damage by insects
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Effects of Fungi on Postharvest Crops
was usually conspicuous, easy to quantify
and these insects were visible to the naked
eye. But, awareness of the losses caused by
fungi, also referred to as ‘moulds’, came with
the discovery of a toxic metabolite called
aflatoxin in 1968 caused by a fungus called
Aspergillus flavus, which killed over 100,000
turkeys in Britain when fed with groundnut
cakes that were infected by this organism.
Studies in Nigeria have revealed the
presence of aflatoxin in Nigerian groundnuts and livestock feed maize; hence, there
is a need to take extremely good care of these
products during storage (Akano and Atanda,
1989). Some crops in which fungal deterioration can take place are as follows: maize,
sorghum, millet, cowpea, beans, groundnut,
cocoa beans, palm kernels and tubers.
Deleterious Effects of Fungi on
Postharvest Crops
Fungi occur everywhere and have a profound
effect on their environment. Like other microorganisms, fungi may be good or harmful,
depending on the species involved. The
deterioration of postharvest crops by fungi
can be either by destruction of the produce
itself or by presenting a potential hazard to
animals or humans. Some of the deleterious
effects of fungi on postharvest crops are as
follows:
29
in Alabama, while Macrophomina phaseoli
causes ‘black mars’ in Gambian groundnuts.
In fruits and seeds, the micropyle is the
common place for infections to begin, but
fungi, bacteria and actinomycetes can develop
in any other region of the seed or fruit, causing abnormal colouring, either localized or
generalized (Clarke, 1968). However, it is
not all discoloration on produce that is
caused by fungi; sometimes, it may be due
to genetic mutations.
Flavour and odour
The flavour and odour of produce caused by
moulds usually affect the taste of the end
products and are not acceptable to consumers. The change in the flavour and odour is
usually as a result of the biochemical change
which takes place in the stored produce.
For example, mouldy groundnuts have
a very unpleasant and sour taste when consumed and these are usually spat out from
the mouth as soon as they are chewed. Undesirable flavour is easily noticed in mouldy
cocoa beans, as it can be detected by tasting
a sample of chocolate which has passed
through all the normal manufacturing processes. Banana and plantain affected by
mould also have a detectable flavour and
odour. Mouldy produce can also have an
odour, ranging from the musty odour of
mouldy grains to the foul smell of rotten
grains (Ogundana et al., 1970).
Discoloration
Fungi come in various colours, i.e. green,
brown, white, grey, black, etc. They impart
these colours on postharvest crops, thereby
changing the original appearance. Discoloured produce is often disliked by consumers and manufacturers in that the colours
affect the end products from such produce.
Cocoa beans, melon seeds, palm kernels,
groundnuts, maize, yam and cassava are
examples of produce in which deterioration
is accomplished by marked discoloration.
For instance, Lasiodiplodia theobromae is
responsible for the disease which discolours
cocoa, widely known as ‘concealed damage’
Biochemical effects
The development of moulds leads to a great
modification in the chemical composition
of the infected produce. One such effect is
an increase in the free fatty acid (FAA) content of the produce. This acid is one of the
intermediate products of spoilage in materials containing fats and oils and its formation results in rancidity. Many of the mould
species infecting our crops are known to
produce lipases, which can hydrolyse fats
into fatty acids by a process called lipolysis,
thereby increasing the free fatty acid content
30
A.O. Ogaraku
of the produce and resulting in a decrease in
oil content and a low protein content.
Kuku (1972) isolated a number of moulds
from palm oil and showed that many of these
increased the FAA of palm oil in pure culture studies. Coursey et al. (1963) isolated a
number of lipolytic fungi from Nigerian
palm kernels. These included A. chevalieri,
A. fumigatus, Paecilomyces variotii and
P. steckii.
The development of mould on produce
causes other modifications; generally, an
increase in reducing sugars and a loss in
protein, which may lead to flours unsuitable for bread making. Moreover, mouldy
rice grain breaks easily during polishing. If
we preserve damp grain in an anaerobic
environment, fermentation results in the
release of carbon dioxide, alcohol and other
volatile substances. The compounds formed
give a bad taste, which remains even after
drying in the open air.
Rotting and caking
Extensive mould activities usually result in
rotting and caking. Rotting and caking renders produce unsightly, decreases milling
yield and quality. Studies carried out by
several workers, including Adeniyi (1970)
and Ogundana et al. (1970), revealed that a
number of fungal species, for example A.
niger, Fusarium moniliforme, P. exalicum,
etc., caused rotting in Nigerian yams. Oyeniran (1970) and other workers carried out
studies that showed that over 30 mould species could be associated with the deterioration of maize in Nigeria.
Weight loss
Fungi growing on plant parts or produce
use them as food substrate. They produce a
variety of enzymes, i.e. amylases, cellulases,
pectinases and lipases, which hydrolyse the
food substances into soluble forms. The
food components easily absorbed and utilized are carbohydrates, proteins, fats and
oil. This breakdown invariably leads to
absolute weight loss. Scientists have reported
up to 10% weight loss in rotting yam tubers
during storage (Ogundana et al., 1970).
Some of the fungi that can cause weight loss
in maize are A. flavus, A. niger, A. candidus, Mucor racemosus and P. pallitans.
Destruction of viability
Fungi reduce the viability of seeds by infecting and destroying their embryo. This in
turn affects the germinability of the seeds
during planting. Broadbent (1967) found
samples of mouldy maize from government
farms in southern Nigeria had only 7–14%
germination, while the mould-free samples
had 100% germination.
Heating
If crops with high water content are piled
up together in one place, heat is generated
and decay sets in. Heating or production of
hot spots is one of the characteristics that
results in rapid fungal development in moist
stored produce, mostly grains and tubers.
Heating in bulk storage is evidence of spoilage
in progress or spoilage already completed.
Growth abnormalities
Groundnut and maize contaminated with
A. flavus produce deformed plants. The
infected young groundnut or maize plant
will have a greatly decreased growth. The
follicles develop poorly and are elongated
in form. During growth, a large number of
sick plants die. Others are continually
abnormal in appearance, while some evolve
into normal plants.
Preparation of the Material for
Attack by Other Agents
It is sometimes difficult in many crops to
separate deterioration or spoilage due to
Effects of Fungi on Postharvest Crops
insects from that caused by fungi, but that
the two are interrelated is in no doubt. What
is in doubt, however, is the exact sequence
of events and the relative damage caused by
the two agents.
Invasion of stored produce by fungi
prepare such commodities for attack by
other agents of deterioration, especially bacteria, insects and mites. In fact, some insects
are known to feed on fungi and in this way
they help to spread the spores. These storage insects can live, develop and reproduce
entirely on certain fungi and thus undoubtedly play an important part as carriers in
the spread of the fungi. An example of such
an insect is Adhasverus advena.
Production of Toxic
Metabolites (Mycotoxins)
since 1960, when it was reported to have
caused the death of about 100,000 turkeys
in Britain when they were fed with groundnut cakes which was infected with A. flavus.
The toxic substance was therefore called
‘aflatoxin’. Different mycotoxins affect different sites of the body. Aflatoxins produced
by A. flavus are the commonest of all the
toxins and affect the liver, causing aflatoxicosis or liver poisoning. High levels of aflatoxin have also been reported to cause
infertility (abnormality in the spermatozoa)
in samples of semen from men fed on diets
contaminated with A. flavus (Ibeh et al.,
1994). The production of aflatoxins on maize
grains and other consumable foods in Nigeria has been reported by many researchers,
including Broadbent (1967), Oyeniran (1970),
Opadokun et al. (1979) and Akano and Atanda
(1989). Other common mycotoxins are:
●
Toxic metabolite production is the most
serious effect of microbiological deterioration of stored products because of its poisoning nature. There are two kinds of
poisoning by fungi, mycetism and mycotoxicosis. In mycetism, the toxic substances
are constituents of the fungi, large enough
to be eaten alone. In mycotoxicosis, the fungus is a contaminant of and has produced
toxic product in some food. The effects of
mycetism include diarrhoea and jaundice,
while mycotoxicoses were defined by Clarke
(1968) as diseases of animals and humans
caused by ingesting poisonous metabolite
fungi that have grown in the food previously before ingestion. Some notable examples of mycotoxicoses are:
1. Ergotism – diseases of cattle in central
Europe caused by the fungi, Claviceps
purpurea.
2. Yellow rice disease of humans in Japan
caused by the fungi, P. citrinum.
3. Alimentary toxic aleukia (ATA) of
humans and cattle caused by F. sporotrichioides.
4. Importantly, aflatoxicosis of poultry and
livestock caused by A. flavus.
This last mentioned toxin disease, aflatoxicosis, has been receiving worldwide attention
31
●
Fumonism – this causes oesophageal
cancer in horses and humans. It is produced by F. graminearum on maize.
Ochratoxin – produced by A. ochareus,
which causes serious nephropathy in
pigs and humans. It is commonly found
in milk and cereals (processed or raw).
Some examples of mould species and the
toxins they produce are shown in Table 3.1.
Conditions that Favour Development
of Fungi on Harvested and
Stored Crops
Fungi, like other living organisms, require
certain conditions for growth and development. These conditions are as follows:
Moisture
It is not the moisture content as such that is
the controlling factor in biological deterioration; it is the relative humidity of the air
in and around the crop. Although relative
humidity is the controlling factor, attention is usually focused on the moisture
content because relative humidity of produce
is difficult to measure, while moisture content
32
A.O. Ogaraku
Oxygen
Table 3.1. Examples of mould species and the
toxins they produce.
Mould species
Toxin produced
Aspergillus flavus
A. ochareus
A. chevalieri
A. nidulans
A. ruber
A. niger
Penicillium islandicum
P. notatum
P. rubrum
P. citrinum
P. patulinium
Fusarium graminearum
Aflatoxin
Ochratoxin
Xanthocillin
Sterigmatocystin
Rubratoxin
Oxalic acid
Islanditoxin
Xanthocillin
Rubratosin
Citrinin
Patulin
Zearalenone
is not. Moisture in stored produce is divisible into two main types: chemically bound
water, which is the part of the intrinsic composition, and physically bound water, some
of which is held loosely on the commodity.
Moisture in terms of water is necessary
for mould spores to germinate, and it also
helps in the process of dissolution of food
materials. The moisture level of a stored
product therefore determines the development rate of the storage fungi.
Mould species vary in their water requirement; for instance, there are those that thrive
at low moisture levels and are said to be xerophytic. Examples are A. flavus, A. chevalieri
and A. repens. Others require high levels of
moisture before they can survive and are said
to be hydrophilic, e.g. Penicillium species.
Most fungi are aerobic; they require oxygen
to survive, like other living organisms. Any
device which cuts off oxygen from the storage environment will reduce, if not totally
eliminate, fungi. This is why storage at an
inert temperature has been effective.
Nutrients
All biological systems, from microorganisms to humans, share a set of nutritional
requirements with regards to the chemicals
necessary for their growth and normal functioning. The great diversity of nutritional
types required are energy, carbon, nitrogen,
sulphur and phosphorus, metallic elements
and vitamins. All these nutritional requirements are present in food substances, such
as carbohydrates, proteins, fats and oil,
which fungi need in soluble forms for metabolic processes.
Fungi produce a variety of enzymes
which break down complex food substances.
Some of these enzymes are as follows:
●
●
●
●
●
Cellulases – break down cellulose in
plant materials.
Amylases – hydrolyse carbohydrates.
Lipases – hydrolyse fats to fatty acids
and glycerol.
Proteases – hydrolyse proteins.
Pectinases – hydrolyse the pectic materials of plant tissues.
Temperature
Heating
All living things have a minimum and maximum temperature for growth. Fungi are a
co-exception. Most fungi will grow at temperatures between 5°C and 35°C. These are
the mesophilic species. There are those that
thrive at 35°C and above and are said to be
thermophilic. Some thrive at very cold temperatures and are said to be psychrophilic.
This means that fungi thrive well in a very
wide temperature range, which gives room
for existence in postharvest crops.
If crops with a high water content are allowed
to overlap or are piled together in one place,
yam for example, heat is generated under
moist conditions and decay will set in.
Insufficient drying
Some crops grow mouldy if insufficiently
dried. Fungi can creep in to destroy the
crops.
Effects of Fungi on Postharvest Crops
33
Preharvest infection
Economic loss
Produce destined for storage is sometimes
infected by moulds before harvest. Most fungi
species also invade, especially following natural or artificial wounds. Some examples are
attack of cocoa beans by Lasiodiplodia theobromae and other moulds, attack of groundnut by Macrophomina phaseoli and attack
of maize by F. moniliforme and P. citrinum.
1. There is a monetary loss because of inaccessibility to foreign trade due to the poor
quality of the produce. There is also a monetary loss because of the poor health of animals fed with inferior feeds.
2. Some fungi, for example Fusarium species, can grow on stored animal feeds, generating products that are highly toxic to swine
and other animals.
3. Infections leading to disease of crops
are extremely important because of the famine, malnutrition and dietary deficiency
they may cause.
4. Some plant pathogens cause food intoxication when eaten by humans or animals;
for example, the fungus, C. purpurea, which
grows on cereal grains and some grasses, replaces the feed kernels with compact masses
of hardened fungus called sclerotia. These
contain alkaloids that act on the nervous
system of humans and other animals, causing gangrene, convulsions and death.
Attack during preparation
During the process of preparation, mould
attacks some produce as a result of lapses in
cultural practices; for example, during
cocoa fermentation mould could infect and
penetrate the beans if the fermenting mass
of beans is not stirred or mixed thoroughly
at intervals. In palm produce, mould can
attack the fruits and sometimes the kernels
when they are heaped on the ground just
before de-husking. In groundnut, the crop
has to be lifted at certain times to avoid
mould contamination.
Control of Fungal Deterioration in
Postharvest Crops
If, for instance, through economy a store is
poorly constructed and the roof is holding
water, it is possible to cause leakage and
water will drip on to the commodity and
thereby cause deterioration.
There are other factors which contribute
to the development of fungi in crops apart
from those mentioned above and they are:
If left uncontrolled, these fungi will cause
deterioration of food products and many
other articles of commerce and industry. For
this purpose, a distinction can be made
between postharvest produce that is stored
dry, such as grains, cocoa, groundnuts, etc.,
and those which are stored with a high water
content, such as yams and other tubers.
However, some of the measures or suggestions listed below will certainly apply to
both types:
1. The degree to which the grain has already been invaded by storage fungi before
it arrives at a given site.
2. The amount of foreign material present
in the grain.
3. The activity of mites and insects. Bored
holes serve as an entry for mould spores.
Some insects, such as A. advena, and mites
feed on mould spores and therefore help to
spread the fungi, as well as increase their
activities in storage.
1. Proper drying of produce to a safe moisture level, either by retaining maize, millet
or guinea corn on the cob and storing in a
condition where gradual drying by heat or
aeration takes place, or otherwise by providing artificial drying.
2. Prevention of damage or wounds on
produce so as to forestall a source of entry
for moulds.
3. Any produce to be stored must be wholesome and healthy. Bruised yam tubers,
Types of stores
34
A.O. Ogaraku
cassava, oranges and fruits should never be
stored.
4. Avoid drying the produce on a bare
floor because of infestation by soil fungi.
5. Hot produce should not be stored. After
drying, allow produce to cool before storage.
6. New produce should not be mixed
with an old consignment, to avoid crossinfestation.
7. Bagged produce should not be placed
on the ground but on raised plank platforms.
8. Overfermentation should be avoided in
produce like cocoa, cassava, etc.
9. The store or warehouse should be leakproof to prevent moisture reabsorption by
the already dried produce.
10. Prevent pockets of heavy insect activity
by proper application of insect control
measure to avoid localized moisture increases and mould growth in the bulk of the
grain.
11. In the case of fruits, harvesting should
be done promptly as very old fruits are
highly susceptible to fungi infection.
12. If possible, dried produce should be
stored in airtight conditions to keep away
from fluctuating atmospheric relative humidity, which could lead to an increase in
moisture content; for example, store in
polythene bags or polythene-lined sacks.
Other methods of controlling deterioration
of dry produce are:
13. Use of fungicides – in the case of
grains not desired for immediate consumption or use, some fungicides such as captan, benomyl, thiobendazole, borax, etc.,
have been used to control fungal attack,
but their use has been limited because of
their toxicity.
14. Use of plant materials – parts or roots
with medicinal properties can also be used
to suppress mould growth in stored crops.
Williams and Akano (1985) reported on the
efficacy of dogonyaro (neem) as a filtrate in
suppressing rotting fungi growth in stored
yam tubers.
15. Addition of chemical preservative
agents – the addition of antiseptics to foodstuffs allows for better preservation under
certain conditions. The use of these products
is subject to regulations in most countries.
Examples of such chemical preservatives
are propionic acid, ascorbic acid, glycerol,
sulphur dioxide and benzoic acid. Their use
in many instances has been limited to livestock feeds.
16. Other technical methods of control –
other methods by which fungal development in stored products can be controlled
are refrigeration, irradiation (for yam) and
storage in airtight containers and inert atmosphere for grains.
Ogundana et al. (1970) found benomyl and
thiabendozole effective in reducing the
activities of fungi in causing yam rot during
storage, but these chemicals are rather toxic.
Research is currently in progress at the
Nigerian Stored Products Research Institute
on the use of safer fungistatic chemicals to
preserve yams against microbiological rot
during storage. Adesuyi (1973) stored yams
successfully for up to 6 months by using a
curing method, cutting off sprouts from
healthy undamaged tubers and using low
temperature and irradiation techniques.
17. Precautions in mycotoxicoses – it is
very important to have a control measure in
harvesting produce in order to eliminate the
fungi causing mycotoxicoses diseases because of their devastating effect on humans
and animals that consume such an infected
crop. Standard safe limits should be determined and enforced levels of aflatoxin and
other toxins in food and feed. Different
countries have a wide variety of tolerance
level of mycotoxin between 5 and 50 µg/kg
(Hansen, 1993). In the USA, the Food and
Drug Association has established an aflatoxin limit of 20 µg/kg for food and feed
ingredients.
A regular monitoring programme should be
arranged for commodities that are susceptible to aflaxtoxin contamination. Processing,
packaging, transportation and storage practices should be well managed to eliminate or
reduce infestation by moulds, especially the
toxigenic strains. Decontamination procedures are to be designed to remove or inactivate the toxins in feed and food. Mycotoxins
can be removed from food by detoxification
using chemical agents.
Effects of Fungi on Postharvest Crops
Conclusions
The role of fungi in the deterioration of postharvest crops is enumerated. The contribution of some workers in providing an insight
into the deleterious effects of fungi on harvested and stored crops, economic loss,
control of fungal deterioration in postharvest crops and precautions in mycotoxicoses diseases is also highlighted. Not forgotten
35
is the most important aspect, the precautions
that need to be taken to control or eliminate
the fungi causing mycotoxicoses in humans
and animals.
It is pertinent to say that knowledge is
far from complete and experts should still
endeavour to find total solutions to the various aspects of these problems as the struggle of humans against the menace of fungi
continues.
References
Adebayo, L.O., Idowu A. and Adesanya, O.O. (1994) Mycoflora and mycotoxins production in Nigeria corn
based snacks. Mycopathologia 126, 183–192.
Adeniyi, M.O. (1970) Fungi associated with storage decay of yam in Nigeria. Phytopathology 60, 590–592.
Adesuyi, D.A.A. (1973) Curing techniques for reducing incidence of rot in yams. Nigerian Stored Products
Research Institute Technical Report No. 12, 57–63.
Akano, D.A. and Atanda, O.O. (1989) The present level of aflatoxin in Nigeria groundnut cake. Letters in
Applied Microbiology 10, 187–189.
Broadbent, J.A. (1967) The micoflora germination and seeding vigour of some maize seeds. Nigerian
Stored Products Research Institute Technical Report No. 15, 113–114.
Clarke, J.H. (1968) Fungi in stored produce. Tropical Stored Product Institute Technical Report 15, 2–14.
Coursey, D.G., Summons, E.A. and Sheridan, A. (1963) Studies on the quality of Nigerian palm kernels.
African Science Association 8,18–28.
Hansen, T.J. (1993) Quantitative testing for mycotoxins in cereal foods. World 38, 346–348.
Ibeh, I.N., Urath and Ogonar, J.I. (1994) Dietary exposure to aflatoxin in human male infertility in Benin City,
Nigeria. International Journal of Fertility and Menopausal Studies 39, 208–214.
Kuku, F.O. (1972) Some mould induced changes in palm kernels. Nigerian Stored Product Research Institute,
Technical Report No. 9, 69–72.
Ogundana, S.K., Haviq, S.H. and Ekundayo, J.A. (1970) Fungi associated with soft rot of yams (Dioscorea
spp) in storage. Nigerian Stored Product Research Institute Technical Report No. 10, 41–45.
Opadokun, J.S., Ikeorah, J.N. and Afolabi, E. (1979) The aflatoxin contents of locally consumed food stuffs.
Nigerian Stored Product Research Institute Technical Report No. 12, 105–108.
Oyeniran, J.O. (1970) Microbiological studies on maize used as poultry and livestock feeds at the research
Farms in Kandan, Western State. Nigerian Stored Product Research Institute Technical Report No. 6,
47–49.
Williams, J.O. and Akano, D.A. (1985) An assessment of wood ash for yam tuber (Dioscorea rotundata) in
storage. Nigerian Stored Product Research Institute Report No. 2, 31–34.
4
Exploitation of Botanicals in the
Management of Phytopathogenic and
Storage Fungi
Pramila Tripathi1 and A.K. Shukla2
1Department
of Botany, D.A.V.-P.G. College, Kanpur, India; 2Department of Botany,
Rajiv Gandhi University, Rono Hills, Itanagar, India
Abstract
Plants are known to contain a number of secondary substances like phenols, flavonoids, quinines,
essential oils, alkaloids, saponins, steroids, etc. Some of these plant-based metabolites have antimicrobial properties and are toxic to phytopathogens. They are also repellant to insects and have fumigant
toxicity against pests. Currently, synthetic pesticides are the primary means of controlling pathogens.
The adverse effects of synthetic pesticides on human health and from the food safety point of view has
enunciated interest in finding an alternative means of controlling phytopathogens and pests. To reduce
dependency on synthetic pesticides, the use of plant-based antimicrobial substances (essential oils,
volatile aromatic compounds, glucosinolates, jasmonates and acetaldehydes) may help in the management of phytopathogens and pests as an alternative method for sustainable agriculture. Use of botanicals is still on a small scale compared to synthetic chemicals; therefore, it is timely to exploit and
formulate low-cost, effective, free of human hazard and eco-friendly plant-based products for the management of pests and pathogens.
Introduction
To control fungal diseases, synthetic fungicides are usually applied as effective, dependable and economical control measures.
However, the indiscriminate use of chemical fungicides has resulted in several problems, such as toxic residues in food, water
and soil and disruption of the ecosystem,
leading to the fear that their regular use may
harm the environment further. Hardly
0.1% of the agrochemicals used in crop protection reach the target pest, leaving the
remaining 99.9% to enter the environment
to cause a hazard to non-target organisms,
including humans (Pimentel and Levitan,
36
1986). According to WHO estimates, approximately 0.75 million people are becoming
ill every year with pesticide poisoning. Further, the resistance of pathogens to fungicides
has rendered certain fungicides ineffective,
giving rise to a new physiological race of
pathogens. Basic research for over more
than 40 years in biology and biochemistry
has made it possible to envisage not only
how new pesticides may be synthesized but
also has generated a completely new approach
to the protection of plants using secondary
plant products which may be toxic to a specific pest yet harmless to humans. Pesticidal
plants have been in nature and its compounds for millions of years without having
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Exploitation of Botanicals
any ill or adverse effects on the ecosystem
and, because of their renewability, they have
a distinct advantage in the management of
disease-causing pests. Plants have a natural
potential to withstand the aggressiveness of
pathogenic species.
Plants synthesize a dazzling array of
structural variety, which inhibits an almost
equally dazzling array of biological activities. A wide spectrum of secondary substances is contained in higher plants,
namely phenols, flavonoids, quinines, tannins, essential oils, alkaloids, saponins and
steroids. The total number of plant chemicals may exceed 4000 and of these, 1000 are
secondary metabolites. These secondary
metabolites have a major defensive role for
plants (Swain, 1977). The search for botanicals from plant species is one of the important areas where Indian scientists can take
a lead and capture the global market. India
enjoys the benefits of a varied climate, from
an alpine climate in Himalaya to a tropical
one in the south and an arid one in Rajasthan to a highly humid climate in Assam
and Bengal. This is consequently reflected
in the rich and diversified flora, which is
often quite distinct, thanks to the natural
barriers that India has all along its frontiers. It is estimated that India has about
17,000 species of angiosperms. There is a
need for extensive screening programmes
at different regional centres of the country
so that knowledge on the various types of
biological properties of angiospermic flora
may be gathered. This type of scientific
testing would definitely be helpful in the
conservation of plant resources and in
proving our sovereign right over our plant
biodiversity. Under these conditions, in any
meaningful search for better and cheaper
substitutes, plant resources for India are a
natural choice. Hopefully, this will lead
to new information on plant application
and a new perspective on the potential use
of these natural products. This chapter
explores the potential to use a variety of
botanicals in the form of plant extracts and
essential oils to control various fungal phytopathogens and fungi related to the storage
of grains and the postharvest pathogens of
perishables.
37
Essential Oils
Essential oils from different plant species
are known to exhibit various kinds of biological activities. The volatility, ephemeral
nature and biodegradability of such volatile
components of angiosperms will be especially advantageous if they are developed as
pesticides (French, 1985). Essential oils are
a complex natural mixture of volatile secondary metabolites isolated from plants by
hydro or steam distillation and by expression. The main constituents of essential oils
are mono- and sesquiterpenes, along with
carbohydrates, alcohols, ethers, aldehydes
and ketones, polyphenolic compounds,
oxides, nitrogen and sulphur compounds
and organic acids, etc. The chemical composition of essential oils is extremely complex and varies with the geographical as
well as the environmental conditions where
the plants are grown (Bhaskara et al., 1998;
Vanneste et al., 2002). The essential oils are
extracted from various parts of plants such
as flowers, fruits, leaves and wood. They are
normally formed in special cells or groups
of cells or as glandular hairs. Oils occur as a
globule or globules in the cell and may also
be excreted from cells lining the schizogenous ducts or canals. They may be present
in glandular regions such as leaves, bark or
fruit and, when occurring in various organs
in one plant, may possess different individual chemical compounds (Bonner, 1991;
Hili et al., 1997). The general antifungal activity of essential oils is well documented (Tripathi et al., 2007, 2008). These essential oils
are thought to play a role in plant defence
mechanisms against phytopathogenic microorganisms (Mihaliak et al., 1991). The emerging picture is that certain specific oils and
their chemical constituents have traditionally been used to protect stored grains and
to repel flying insects in the home and have
demonstrable contact and fumigant toxicity
to a number of economically important insects
and mite pests, as well as to pathogenic fungi.
The essential oils or their major constituents could be effective fumigants and also
could be integrated with other pest management programmes. Natural pesticides based
38
P. Tripathi and A.K. Shukla
on plant essential oils could represent alternative crop protectants. The essential oils
produced by different plant species are, in
many cases, biologically active and have
antimicrobial, allelopathic, antioxidant and
bioregulatory properties (Caccioni and Guizzardi, 1994; Vaughan and Spencer, 1994).
Sometimes, the chemicals in the oil, as well
as the oil itself, are registered as pesticide
active ingredients. It is also fairly common
for two or more oils to be used in the same
commercial product. Since the essential oils
as such are a mixture of different major and
minor components which act synergistically
in the biological efficacy of the oil, there
would be less chance of the development of
physiological races of the target pathogens
if the oils as such were formulated as botanical pesticides and fumigants. Essential oils
as botanical pesticides may be produced
easily, even by small-scale industries, as
there is no sophisticated procedure for their
distillation and most aromatic plants are
available locally. They thus constitute a
friendly, natural alternative in pest control.
Essential Oils Against
Phytopathogenic Fungi
The antifungal activity of essential oils has
been studied by a number of workers
(Apablaza et al., 2004; Harish et al., 2004;
Muller-Ribeau et al., 1995). Singh et al.
(1980) found that essential oils from Cymbopogon spp. and Trachyspermum ammi L.
exhibited strong antifungal activity against
Bipolaris oryzae. Carvone, a monoterpene
isolated from the essential oil of Carum
carvi, was found to inhibit the sprouting of
potatoes during storage.
Carvone was also found to have fungicidal activity that helped to protect potato
tubers from fungal rotting without exhibiting
mammalian toxicity (Hartmans et al., 1995).
It has been introduced in the Netherlands
under the trade name TALENT. Besides, the
essential oils of Salvia officinalis have also
shown practical potency in enhancing the
storage life of some vegetables by protecting them from fungal rotting (Bang, 1997).
Powdery mildew of Cucurbita maxima is
caused by Sphaerotheca fuliginea. Reynoutria extracts and olive oil were found to be
effective in controlling the disease (Cheah
and Cox, 1995). Since olive oil is used in
cooking, food additives and medicines, it
does not cause any human health or environmental problems. Recent studies in
Ghana confirm that Ocimum gratissimum
and Syzigium aromaticum are very effective
in preventing fungal growth (FAO, 1999).
Essential Oils Against Fungal
Pathogens of Seeds
The fungicidal effect of essential oils against
pathogens of cereal grains has been tested
successfully. It is especially significant in
the case of stored rice, where currently fungicides are not used to control fungal pests.
Peppermint (Mentha piperata), thyme (Thymus copitatus) and caraway (C. carvi) oils
have demonstrated effective control against
fungal pathogens like Fusarium sp., Macrophomina phaseolina and Colletotrichum
dematium (Abdelmonem et al., 2001). Essential oils from oregano (Origanum vulgare)
and thyme were applied as fumigants against
the mycelia and spores of Aspergillus flavus,
A. niger and A. ochraceus infesting wheat
grains. Only oregano essential oil exhibited
fungicidal activity (Paster et al., 1995). The
antifungal activity of the essential and fixed
oils of thyme, clove, peppermint, soybean
and groundnut were tested against A. flavus, A. niger, F. oxysporum, F. equiseti and
Penicillium chrysogenum in vitro on the
cowpea (Vigna unguiculata) (Kritzinger et al.,
2002). Thyme and clove oils inhibited growth
of all the fungi significantly at concentrations of 500 and 1000 ppm. Peppermint oil
inhibited growth of the above-mentioned
fungi successfully at 2000 ppm (Kritzinger
et al., 2002). In blackgram (V. mungo), essential oil extracted from wood chips of cedar
(Cedrus deodara) and that from seeds of T.
ammi exhibited antifungal activity, inhibiting the mycelial growth of A. niger and Curvularia ovoidea, two storage fungi found on
seeds (Singh and Tripathi, 1999). A. flavus
Exploitation of Botanicals
was also found infesting seeds of guar
(Cyamopsis tetragonoloba), a native plant of
India which has main commercial value
due to its seed gum (galactomannan gum).
In this case, A. flavus was controlled by
cumin (Cuminum cyminum L.) oil extracted
from its seeds (Dwivedi et al., 1991). Chemical studies indicated that the greater part
of this antimicrobial activity might be
attributed to the cuminaldehyde that is
present in the dried fruit of this plant (De
et al., 2003). The essential oils of Cassulia
allaris and M. arvens have been reported as
botanical fumigants for management of the
biodeterioration of wheat from A. flavus
(Varma and Dubey, 2001).
Essential Oils Against
Aflatoxicogenic and
Mycotoxicogenic Fungi
The aflatoxins are well known for their carcinogenic, mutagenic and teratogenic effects
on humans and domestic animals (Wyllie
and Morehouse, 1978). A natural fungicide
against aflatoxigenic fungi to protect stored
rice using the essential oil of lemongrass (C.
citrates) was developed by Paranagama et al.
(2003). Lemongrass oil was tested against
A. flavus and the test oil was fungistatic
and fungicidal against the test pathogen at
0.6 and 1.0 mg/ml, respectively. Aflatoxin
production was inhibited completely at
0.1 mg/ml. Citral has been found as a fungicidal compound in lemongrass oil. During
the fumigant toxicity assay of lemongrass
oil, the sporulation and mycelial growth of
the test pathogen were inhibited at a concentration of 2.80 and 3.46 mg/ml, respectively.
Lemongrass oil could be used to manage aflatoxin production and to inhibit the fungal
growth of A. flavus in stored rice.
Putative mycotoxicogenic fungi were
partially or completely sensitive to different
essential oils extracted from different medicinal plants (Soliman and Badeaa, 2002). Seed
treated with cinnamon, palmarosa and lemongrass oils at 500 mg/kg showed antimycotoxigenic ability against fumonisin B1
accumulation produced by F. vesticillioides
39
and F. proliferatum (Marin et al., 2003).
Velutti et al. (2004) reported antimycotoxicogenic activity of the essential oils against
F. graminearum infested seeds. The essential oils of oregano, cinnamon, lemongrass,
clove and palmarosa effect the growth rate
of F. graminearum and mycotoxin Zearalenone (ZEA) and Deoxynivalenol (DON) production at two concentrations (500 and
1000 mg/kg).
Plant Extracts Against
Phytopathogenic Fungi
The preservative nature of some plant
extracts has been known for centuries and
there has been renewed interest in the antimicrobial properties of extracts from aromatic plants. The application of the extracts
of higher plants to control plant diseases was
first attempted by Democritus as early as
470 BC. Plant extracts have assumed special significance nowadays as an eco-friendly
method for plant disease management. Plants
contain alkaloids, tannins, quinines, coumarins, phenolic compounds, phytoalexins and ipomeamarone in the extract,
which are known for their antifungal property (Datar, 1999). Use of plant extracts for
seed treatment is one of the alternative
methods of preventing pathogen problems
of agricultural crops. Plant materials as
such can be used as soil amendments that
can serve as both a nutrient as well as an
antifungal agent. Plant extracts have also
been reported to stimulate the growth of
targeted plant species. This is probably due
to some hormones and allied substances
like IAA, IBA, etc.
However, the active principles of some
plants have been isolated phytochemically
and have shown a strong inhibitory action
against a number of fungi. Antifungal activity of plant extracts against a wide range of
fungi has been reported by a number of
workers (Grange and Ahmed, 1988; Davidson and Parish, 1989). Bhargava et al. (1981)
screened extracts of some plant species and
found O. canum to be most effective against
A. flavus and A. versiolor. Pandey et al.
40
P. Tripathi and A.K. Shukla
(1982) evaluated the seed extract of 30 plants
and found soybean, Leonotis nepetaefolis,
Parpalum and Peltophorum to exhibit an
inhibitory effect against the fungi, Alternaria
alternata and A. niger. Ark and Thompson
(1959) found the leaf extract of Allium sativum to be effective against various plant
pathogens. Acacia nilotica (leaf and bark)
and A. farnasiana (bark) of Mimosaceae
showed high activity, while A. catechu of
the same family did not show activity either
from leaf or from bark (Tripathi, 2005). Four
compounds, i.e. iritin A, iritin B, flavononedehydroulogonin and sesquiterpene pygmol, were isolated with dichloro-methane
extract of the aerial parts of Chenopodium
procerum. These compounds have been
found to inhibit the growth of the plant
pathogenic fungi, Cladosporium cacumerinum (Bergeron et al., 1995). Kim et al.
(2004) evaluated Achyranthus japonica and
Rumex crispus for activity against various
plant pathogenic fungi and control of powdery mildew. Methanol extract of the fresh
material of 183 plants was screened in vivo
for antifungal activity against Magnaporthe
grisea, Corticium sasaki, Botrytis cinerea,
Phytophthora infestans, Puccinia recondita
and Erisiphe graminis. Among them, 33 plant
extracts showed disease control efficacy. The
methanol extract of Achranthes japonica
(whole plant) and R. crispus (roots) at a concentration greater than 11 g fresh weight of
plant tissue per litre aqueous Tween 20
solution controlled the development of barley powder mildew caused by E. graminis
effectively in an in vivo assay using plant
seedlings. Some fungi like F. solani and
Verticillium alboatrum have been shown to
be susceptible to tannins extracted from the
bark of various trees, including chestnut
and wattle (Lewis and Papavizas, 1967).
The effects of aqueous and methanol, petroleum ether, chloroform and ethyl acetate
extracts of Cyprus rotundus were tested on
spore germination of F. solani. Ethyl acetate
extract exhibited an inhibitory effect on
spore germination at 1000 µg/ml (Singh and
Tripathi, 1999). In the field, reduction of
disease incidence has been recorded as a
result of plant seed treatment with extract,
and an increase in yield was also noted.
Plant Extracts in the Management
of Fungal Seed Diseases
Cereal seeds carry a wide range of fungi that
are known to play a significant role in spoilage and probably rank second only to insects
as a cause of deterioration and loss in all
kinds of field and storage crops throughout
the world (Christensen and Kaufman, 1974).
The information on fungal association with
important cereal grains is relevant in assessing the potential risk of mycotoxin contamination. In recent years, the use of plant
extracts for controlling fungal seed disease
has also been of renewed interest. Carvone
(monoterpene compound) completely inhibited F. oxysporum and A. pisi. African yam
bean, Sphenostylis stenocarpa, is an important grain legume in most tropical African
countries (Nwachukwu and Umechuruba,
2001). Major pathogenic fungi associated
with this crop are A. niger, A. flavus, Lasiodiplodia theobromae and F. moniliforme.
Associated fungi could be controlled by
using crude and aqueous extract of basil (O.
basilicum), bitter leaf (Vernonia amydalina), neem and pawpaw (Carica papaya).
Parimelazhagan and Francis (1999) reported
reduction in the radial growth of Curvularia
lunata associated with rice seeds when
treated with leaf extract of Clerodendrum
viscosum, which also increased seed germination, root and shoot length of the rice.
The same results were observed by using
plant extracts to control B. oryzae on rice
seeds, which have a high natural infection
of the fungus (Alice and Rao, 1986). In Bangladesh, use of the extract of Polygonum
hydropiper, A. cepa, A. sativum and A. jidia
demonstrated to be effective against B.
oryzae at higher concentrations. Among
them, neem and garlic were the most effective at 1:1 dilution and inhibited the occurrence of the pathogen by 91 and 83%,
respectively (Ahmed et al., 2002).
Alternaria padwickii, another important seedborne pathogen of rice, was also
inhibited by aqueous extract of Strychnos
nux-vomica, garlic bulbs, ginger rhizome,
basil leaves and fruits of A. indica (Shetty
et al., 1989). The ability of natural plant
Exploitation of Botanicals
extracts to prevent the growth of fungi naturally infesting grains was also studied. Before
sowing, wheat seeds were soaked in an
aqueous plant extract of O. gratissimum and
disease transmission was evaluated. The
rate of infection decreased with the extract
at concentrations higher than 10% (Rodrigues et al., 2001). Leaf extracts of Delonix
regia, Pongamia glabra and A. nilotica significantly inhibit spore germination, mycelial growth and spore production of A.
helianthi, M. phaseolina and F. solani from
sunflower seeds (Tribuhavanaamala and
Narsimhan, 1998). Melon seeds are very
important as as condiment and constitute a
very valuable source of oil and protein for
many people of West Africa (Oyolu, 1977).
After 6 months of incubation, all the melon
seeds treated with leaf extract showed no
infection except M. phaseolina. Ahmad and
Prasad (1995) evaluated that post-infection
treatment of sponge-gourd fruits with the
extracts of Azadirachta indica, Lantana
camara, Murraya exotica, O. sanctum,
Datura fistulosa and Catharanthus roseus
almost fully inhibited the spread of disease
caused by Helminthosporium spiciferum
and F. scirpi.
Application of Botanicals
in Seed Storage
Quality seed should have higher vigour and
viability and these two characteristics cannot be maintained in storage because they
deteriorate rapidly under storage conditions
and suffer quantitative and qualitative losses
due to pests and diseases. Therefore, treating seeds with synthetic chemicals is vital
for successful storage. However, these chemicals are hazardous to humans. Therefore,
use of natural plant products for long-term
seed storage has multi-purpose benefits as
eco-friendly protection against the ageing
process, prevention of insects and fungi and
for their cost effectiveness (Vanangamudi
et al., 2007). During storage, the enzymatic
activity (amylase, catalase, peroxidase,
superoxide dismutase and dehydrogenase)
responsible for maintenance of seed quality
41
and the antioxidant (ascorbic acid) contents
are maintained at optimum level in botanically treated seeds (Umarani, 1999). Common botanicals, arrapu (Abizia amaru), neem
(A. indica), notchi (Vitex negundo), Prosopis
sp., pungam (Pongamia glabra), moringa
and tamarind, contain an auxin-like substance which regulates seedling growth
and initial establishment. In botanicals, a
gibberellin-like substance is also present in
addition to saponin and other nutrients,
which interact with amino acids, tryptophane to form the indole acetic acid (IAA),
which leads to release of plant hormones
that are responsible in cell elongation and
vegetative growth. In botanical seed pelleting, the leaf powder acts as a water pad by
absorbing/regulating soil moisture availability, which enhances a better seed–soil relationship (Narasimha, 1994). Seeds are stored
by pelleting them with botanical products.
The aim of botanical pelleting in seed storage is to extend storage potential, besides
maintaining its ability to produce normal
seedlings. Jegathambal (1996) found that
sorghum seeds hardened and pelleted with
arappu leaf powder could be stored for
up to 2 weeks with higher germinability.
Papaya seeds pelleted with botanicals or
presoaked with botanicals gave improved
germination, vigour index and field emergence when compared to the control or
water socking (Ananthakalaiselvi, 1995).
Dry dressing of seeds with botanicals prolongs the storability of the seeds in many
crops, especially in pulses, and acts as a
dual-purpose technologically for seed storage by preventing biotic organisms attacking the seeds during storage. Sabir (1989)
reported that soybean seeds treated with
sambangi (Polianthes tuberose) seed powder at a ratio of 1:100 maintained a higher
germination rate (70%), even up to 8 months
after storage. Pea seeds dried and mixed
with notchi (V. negundo) powder or sambangi seed powder at a ratio of 1:100 maintained a higher germination rate after up to
8 months in storage (Paramasivam, 1990).
Umarani (1999) reported that dressing
dried Casuarina seeds with neem leaf powder extended the storability of the seeds for
up to 9 months.
42
P. Tripathi and A.K. Shukla
Biocide Formulation of Essential Oils
The formulation of plant metabolites must
be introduced to overcome their degradation and to be used practically during handling and application as biocides. Such
formulation could be used easily and
diluted with water to form the appropriate
concentrations in different applications.
Study should be continued to evaluate the
pesticidal activity of the produced formulated biocides against some plant pathogenic microorganisms. Narsimhan et al.
(1988) demonstrated that neem oil (A. indica)
and pungam oil (P. pinnata) emulsifiable
concentrate formulation prevented sheath
rot (Sarocladium oryzae) of rice. Gascon
et al. (1999) showed that the essential oils
of rosemary, jarilla, mendocina, tomillo
mendocina, origanum, tarragon, lavandins
and eucalyptus were emulsified with different formulations of water suspensions of
wall support systems using both a handheld propeller blender and a high pressure,
double effect homogenizer. Also, Bowers
and Locke (2000) report that several commercial formulations of botanical extracts
and essential oils have been investigated as
possible alternatives for soil fumigation to
control Fusarium wilt disease. Essential oils
of fennel, peppermint and caraway have
been formulated in the form of stable emulsifiable concentrates.
occurrence as part of the diet, their ephemeral
nature and their biodegradability suggest
low toxic residue problems. Such compounds
could be extracted and applied to other
harvested perishables. Some of the volatile
aromatic components, namely acetaldehyde, 6-carbon (C6) aldehydes, benzaldehyde, hexenel and hexanal, are of significant
importance.
Aldehydes
Vapours of acetaldehyde have been used to
control B. cinerea (Prasad and Stadelbacher,
1973). Avissar and Pesis (1991) reported
acetaldehyde to be active against B. cinerea
and Rhizopus stolonifer causing rot to
strawberry fruits. Benzaldehyde has been
used in the laboratory to fumigate peaches
and to protect them against Rhizopus rot. It
inhibits spore germination of B. cinerea
totally at 25 µl/l and germination of Monilinia
fructicola at 125 µl/l (Wilson et al., 1987).
The aldehydes, benzaldehyde, acetaldehyde
and cinnamaldehyde, ethanol and benzyl
alcohol were found to be the strongest
growth inhibitors and the most lethal to fungal spores and mycelia of fruit and vegetable
pathogens like P. digitatum, R. stolonifer and
Colletotrichum during in vitro trials.
Hexenal and hexanal
Botanicals in the Management of
Postharvest Diseases of Perishables
Botanicals as antifungal agents
in postharvest disease control of fruits
Fruits and vegetables have a number of constituents and inducible volatile aromatic and
flavour compounds (Tripathi, 2007). These
aromatic and flavour components are generally produced by fruits during ripening
and provide resistance to the fruits at the
postharvest stage. The flavour compounds
are secondary metabolites having unique
properties of volatility and low water solubility. As potential fungicides, their natural
(E)-2-Hexenal and hexanal are two different
volatile flavour compounds. Hexenal vapours
have a number of attributes that may be
important in consumer demand for more
natural measures to combat fruit diseases
with fewer toxic residues. Hexenal vapour
inhibited hyphal growth of apple slices
(Song et al., 1996). Archbold et al. (1999)
showed (E)-2-hexenal to be an efficient fumigant in controlling mould on ‘Crimson Seedless’ table grapes. (E)-2-Hexenal has been
found to be strongly antifungal in nature
and its in vitro and in vivo activity against
B. cinerea has been reported by a number of
workers (Hamilton-Kemp et al., 1992; Fallik
et al., 1998). The effect of trans-2-hexenal
Exploitation of Botanicals
on the control of blue mould disease (P.
expansum) in reducing patulin content and
on improving the fruit quality of ‘Conference’ pears has been evaluated and greater
reduction of decay was obtained by treatment at 12.5 µl/l at 20°C for 24 or 48 h after
inoculation (Neri et al., 2006).
Acetic acid
Acetic acid is a metabolic intermediate that
occurs naturally in many fruits (Nursten,
1970). There are several advantages in using
acetic acid fumigation. It is a natural compound found throughout the biosphere,
posing little or no residual hazard. Low concentrations, i.e. 2.0 or 4.0 mg/l, of acetic acid
in air have been found to be extremely effective for controlling B. cinerea conidia on
apple (‘Red Delicious’) fruit (Sholberg and
Gaunce, 1995). Acetic acid has been shown
to be an effective fumigant for commercial
use on apricot and plums (Liu et al., 2002),
grapes (Sholberg et al., 1996) and sweet cherries (Sholberg, 1998; Chu et al., 1999, 2001).
The use of acetic aid and vinegar is the better
choice in most cases because it does not have
an objectionable odour and has a long history of use on food (Sholberg et al., 2000).
Jasmonates
The term ‘jasmonates’ includes jasmonic
acid (JA) and methyl jasmonate (MJ). These
are naturally occurring plant growth regulators that are widely distributed in the plant
kingdom and are known to regulate various
aspects of plant development and responses
to environmental stresses (Sembdner and
Parthier, 1993; Creelman and Mullet, 1995,
1997). Droby et al. (1999) found that postharvest application of jasmonates reduced decay
caused by grey mould, P. digitatum, either
after natural or artificial inoculation of ‘Marsh
Seedless’ grapefruit. When applied at low
concentrations, jasmonates are potential postharvest treatments to enhance natural resistance and to reduce decay in fruit. Since they
are naturally occurring compounds and are
43
given in low doses, jasmonates may provide
a more environmentally friendly means of
reducing the current chemical usage.
Glucosinolates
Among natural substances with potential
antimicrobial activity are the glucosinolates,
a large class of approximately 100 compounds produced by members of the family
Crucifereae, with well-documented activity
(Fenwick et al., 1983). Hydrolysis of glucosinolates produces D-glucose, sulphate ion
and a series of compounds such as sothiocyanate (ITC), thiocyanate and nitril. The
antifungal activity of six glucosinolates has
been tested on several postharvest pathogens, namely B. cinerea, R. stolonifer, M.
laxa, Mucor piriformis and P. expansum,
both in vitro (Mari et al., 1993) and in vivo
(Mari et al., 1996). Allyl-isothiocyanate
(AITC), a naturally occurring flavour compound in mustard and horseradish, has a
well-documented antimicrobial activity.
Exposure of pear fruit to an AITC-enriched
atmosphere resulted in good control of blue
mould, including a TBZ resistant strain on
pears (Mari et al., 2002). The use of AITC,
produced from purified sinigrin or from
Brassica juncea, against P. expansum appears
very promising as an economically viable
alternative with moderately low impact on
the environment.
Essential oils
The antimicrobial effects of essential oils
(EOs) or their constituents on postharvest
pathogens have been studied quite extensively (Bishop and Thornton, 1997; Tripathi
et al., 2007). The advantage of EOs is their
bioactivity in the vapour phase, a characteristic that makes them attractive as possible
fumigants for stored product protection.
Control of the storage pathogen, B. cinerea,
on Dutch white cabbage (B. oleracea var.
capitata) by the EOs of Melaleuca alternifolia in in vitro conditions has been investigated (Bishop and Reagon, 1998). Tripathi
44
P. Tripathi and A.K. Shukla
et al. (2008) evaluated some EOs against
moulds of grapes caused by B. cinerea. The
effect of C. nardus EO on the growth and
morphogenesis of A. niger has been tested
(Bellerbeck et al., 2001). The potential of
using EOs by spraying or dipping to control
postharvest decay has been examined in
fruits, namely cherries, citrus fruits, apple,
peaches and cabbage (Tiwari et al., 1988;
Smid et al., 1994; Dixit et al., 1995). Thymol
is an EO component from thyme (T. capitatus). Fumigation of sweet cherries with thymol was effective in controlling postharvest
grey mould rot caused by B. cinerea (Chu
et al., 1999) and brown rot caused by M.
fructicola (Chu et al., 2001). The shelf life
and safety of some perishable foods treated
with EOs have been improved remarkably
(Ponce et al., 2004; Holley and Patel, 2005).
The EO of S. officinalis has also shown
practical potency in enhancing the storage
life of some vegetables by protecting them
from fungal rot (Bang, 1995). Treatment of
oranges by fumigation with the EOs of M.
arvensis (100 µl/l), O. canum (200 µl/l) and
Zingiber officinale (200 µl/l) has been found
to control blue mould, thereby enhancing
shelf life (Tripathi et al., 2004). Plaza et al.
(2004) evaluated the potential of thyme,
oregano, clove and cinnamon EOs against P.
digitatum and P. italicum on citrus fruits.
The postharvest quality of strawberry and
tomato fruit was evaluated after treatment
with Eucalyptus and cinnamon volatile EO
vapours (Tzortzakis, 2007).
Plant extracts
Some plants extracted in different organic
solvents have shown inhibitory action
against different storage fungi (Singh et al.,
1993; Hiremath et al., 1996; Rana et al.,
1999; Okigbo and Pandalai, 2005). The
inhibitory effect of water-soluble extracts of
garlic bulbs, green garlic, green onions, hot
peppers, ginger, Chinese parsley and basil
on the growth of A. niger and A. flavus was
examined. Garlic bulbs, green garlic and
green onions showed an inhibitory effect
against these two fungi (Yin and Cheng, 1998).
Treatment of pineapple fruits infested with
C. paradoxa by X. strumarium extract reduced
the severity of the disease (Damayanti et al.,
1996). The phytochemical investigation of a
methanolic extract of A. nilotica resulted in
isolation of kaempferol. It has shown antifungal activity against P. italicum at 500 µg/l
(Tripathi et al., 2002). In vitro inhibition of
B. theobromae causing Java black rot in
sweet potato was induced by phenolic compounds, chlorogenic acid giving the highest
in vitro inhibition, followed by pyrogallol,
pyrocatechol, phenol and resorcinol. Low
concentrations of phenols are required by
the fungus during normal metabolism, but
higher concentrations are inhibitory to growth
(Mohapotra et al., 2000). The phytochemical
investigations of most plants have resulted
in the isolation of active principles. These
compounds when tested against postharvest
fungi have shown pronounced antifungal
activity. A naturally occurring compound
isolated from the flavedo tissue of ‘Star Ruby’
grapefruit (Citrus paradise) identified as
7-geranoxy coumarins exhibited antifungal
activity against P. italicum and P. digitatum
during in vitro and in vivo tests (Agnioni et al.,
1998). Arya (1988) controlled fruit rots by leaf
extracts of medicinal plants.
Mode of Action of Essential Oils
The mechanism of action of EOs and other
bioactive phytocompounds against microorganisms is a complex process and has
not yet been fully explained. It is generally
recognized that the antimicrobial action of
essential oils depends on their hydrophobic
or lipophilic character. Terpenoids may
serve as an example of lipid-soluble agent
that affects the activities of membrane catalysed enzymes; for example, their action on
respiratory pathways. Compounds of EOs
either affect the physiological function of
microorganisms or cause structural changes
of hyphae and spores (Thompson, 1986;
Arras et al., 1993; Zambonelli et al., 2004).
For instance, the effect of thyme oils and
thymol on the hyphae cytomorphology of
F. solani, R. solani and C. lindemuthianum
Exploitation of Botanicals
increased vacuolization of the cytoplasm
and accumulation of lipid bodies, undulation of the plasmalemma and alteration of
the mitochondrial and endoplasmic reticulum (Zambonelli et al., 2004). However,
variations in the fungicidal action of the
compounds seem to depend on solubility,
as well as on the capacity to interact with
cytoplasmic membrane.
Conclusions
Sustainable agriculture in the 21st centaury
will rely increasingly on alternative interventions for pest management that are environmentally friendly and reduce the amount
of human contact with chemical pesticides.
The use of botanicals in crop protection has
now gained popular ground in the world of
agriculture as an alternative to the use of
toxic, persistent and synthetic compounds.
Several factors are now responsible for making the use of alternative methods more
attractive. A number of studies have been
conducted on the use of botanicals and several plants with promising biocidal properties have been identified. Most of these
plants have also been used in vitro and in
vivo in the control of various plant diseases.
Certain plant EOs and plant extracts have a
broad spectrum of activity against plant
pathogenic and other fungi. They have considerable potential as crop protectants. Current information indicates that they are safe
to the user and the environment, with few
qualifications. With the modern techniques
now available and the attention being given
to this area, we look forward to intensifying
development of the biological activity of
botanicals so as to exploit them as fungicides. A consolidated and continuous search
for natural products may yield safer alternative control measures like azadirachtin and
pyrethroids, which are being used. However, in order to consider the use of any
plant material seriously, further information is required. The use of locally available
plants avoids the need to establish complex
mechanisms for pesticide distribution; the
community can collect or grow the plants
45
itself. Effective antifungal plant compounds
that can fill the void of phased-out chemicals will require some advances in the study
of regional aromatic plants, their production, formulation and possible beneficial
mechanism to prevent or control fungal
attack, better understanding of how they
will fit into integrated systems and their
interaction with the environment and other
IPM components and identification of the
optimum concentration of EOs that can control seedborne fungal pathogens without
affecting seed germination and seedling
growth. The information on the active principles present in various botanicals on germination and seedling vigour is to be
elucidated. A proper study of the mode of
action and structure activity relationship
will bring about a new class of interesting
compounds for future pest control. Despite
the common belief that phytocompounds
are safe, they all have inherent risk, just like
synthetic compounds. Thus, it is within the
scope of phytoscientists to elucidate the
side effects and appropriate doses and identify bioactive phytocompounds and ways of
extraction and conservation. As a cautionary
note, the EOs that are the most efficacious
against pests are often the most phytotoxic.
This phytotoxicity requires serious attention when formulating products for agricultural use. Also, selectivity among invertebrates
is not well documented. Honeybees appear
somewhat susceptible (Lindberg et al., 2000).
The susceptibility of various natural enemies has yet to be reported, although the
lack of persistence of EOs under field conditions could provide some information on
temporal selectivity favouring non-target
species. Finally, we should maintain our
efforts in considering and valorizing our
natural patrimony, as well as conducting
more scientific research on aromatic plants
for chemical analysis and biological, toxicological and pharmacological investigation
of therapeutic aspects. It is important to
remember that just because a pesticide is
derived from a plant does not mean that it is
safe for humans and other mammals, or that
it cannot kill a wide variety of other life.
Some botanical pesticides can be quite toxic
to humans and should not be used on plants
46
P. Tripathi and A.K. Shukla
for human consumption. For example, methyl
salicylate (oil of wintergreen) is commonly
used as food flavouring, but it can be quite
toxic in large doses (Jonathan and Davis,
2007). Few systematic studies have been conducted to determine how farmers use plant
protectants, their effectiveness and method
of application. The introduction of rapid
rural appraisal (RRA) and participatory rural
appraisal (PRA) techniques will facilitate the
collection of this type of information. For
utilization of botanicals on an industrial
scale, it may be necessary to obtain such
secondary metabolites from tissue culturederived materials. There are many advantages to this method of production, including
immediate response to an increase in demand
irrespective of season, freedom from climatic
stresses, pests and diseases and product formation in a clear, sterile environment.
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Research 16, 69–74.
1
2
3
4
5
Plate 1. (a) Perithecia of Gibberella zeae (anamorph Fusarium graminearum) on infected seed of triticale.
(b) Cross-section of a perithecium of G. zeae showing the ostiole and asci bearing ascospores. (Reprinted with
permission from F. Trail and R. Common (2000). Perithecial development by Gibberella zeae: a light microscopy
study. Mycologia 92,130-138. © Mycological Society of America)
Plate 2. Diaporthe phaseolorum (anamorph Phomopsis sojae) causing seed rot on soybean seeds. (Courtesy M.
C. Rollán)
Plate 3. Fusarium sp. Infecting soybean seeds. (Courtesy M. C. Rollán)
Plate 4. Germinating onion seed affected by Botrytis allii. (Courtesy L. du Toit, Diseases in vegetable seed crops:
Identification, biology, and management [Online]. Available at: http://www.seedalliance.org/uploads/pdf/VegSeedDiseases.pdf)
Plate 5. Seedborne wilt of spinach by Verticillium dahliae. (Courtesy L. du Toit)
6
7
8
9
10
Plate 6. Spinach seed showing stromatisation due to pseudothecia of Pleospora herbarum (anamorph Stemphylium botryosum). (Courtesy L. du Toit)
Plate 7. Rice seed discoloration caused by a fungi complex.
Plate 8. Wheat seed discoloration caused by a fungi complex.
Plate 9. Open pod of soybean showing purple discoloration caused by Cercospora kikuchii. (Courtesy M. C. Rollán).
Plate 10. Conidiophores and conidia of Cladosporium variabile on spinach seed. (Courtesy L. du Toit)
5
Use of Plant Extracts as Natural
Fungicides in the Management of
Seedborne Diseases
Gustavo Dal Bello and Marina Sisterna
Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Centro
de Investigaciones de Fitopatología (CIDEFI) – Facultad de Ciencias Agrarias y
Forestales, Universidad Nacional de La Plata, La Plata, Argentina
Abstract
Seedborne fungi can cause substantial losses to grains, rendering them unfit for human consumption
and sowing. Several methods have been used for the control of seedborne diseases and among them
chemical control has been the most widely adopted over many decades. The use of most of these fungicides has been restricted because of high and acute toxicity, long degradation periods and bad effects
on human health, plants and animals, which is harmful to our environment. Moreover, recent increases
in the production and sale of organic seed has heightened the scrutiny of organic seed quality and in
particular brought attention to concerns of seedborne disease contamination. In order to meet the
demands of consumers and growers alike, exploration of alternative methods for managing fungal diseases is under way. One such eco-friendly approach of controlling seed fungal diseases is the use of
natural products, specifically plant-derived compounds. They have played a significant role in reducing the incidence of seedborne pathogens and in the improvement of seed quality and the emergence
of plant seeds in the field. It has long been recognized that several plant compounds, such as essential
oils, have antifungal activity against both pathogens and spoilage fungi. As a rich source of bioactive
chemicals, plants may provide potential alternatives to synthetic fungicides for seed treatment to protect them against seedborne pathogens. Therefore, this chapter discusses the current status of the use
of plant extracts to control seedborne fungi.
Introduction
Almost 90% of all the world’s food crops
are grown from seeds (Schwinn, 1994), which
are widely distributed in national and international trade. Many plant pathogens can
be seed transmitted and seed distribution is
a very efficient means of introducing plant
pathogens into new areas, as well as a means
of survival of the pathogen between growing seasons. Disease-causing organisms may
be carried with, on or in seeds and, in suitable environmental conditions, may be
transmitted to cause diseases in developing
seedlings or plants. With some diseases, the
pathogen attacks the germinating seedling,
which affects seedling establishment and
hence plant populations; with others, disease symptoms are not seen until a later stage
of growth (Rennie and Cockerell, 2006). Furthermore, seedborne pathogens such as bacteria, fungi, viruses and nematodes have the
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
51
52
G. Dal Bello and M. Sisterna
potential to spread disease to the subsequent
crop. Seedborne infection of fungal pathogens is important not only for its association
with the seeds but also contamination of the
soil by permanently establishing its inocula.
Additionally, fungi are significant destroyers of foodstuffs and grains during storage,
rendering them unfit for human consumption by retarding their nutritive value and
often by producing mycotoxins (Satish et al.,
2007).
It is, therefore, necessary to search for
control measures that are economical, ecologically sound and environmentally safe to
eliminate or reduce the incidence of these
important pathogens so as to increase seed
germination and obtain healthy and vigorous plants with better yield (Hasan et al.,
2005).
Seed treatment is the oldest practice in
plant protection. Its origin can be traced to
the 18th century with the use of brine to
control cereal smuts (Neergaard, 1979). The
modern era of seed treatments began with
the introduction of organomercury fungicides in 1912, which were widely used for
several decades. The post-World War II
period saw the development of new fungicide chemistry and the first use of seed
treatment for insect control. Today, the most
widely used application of seed treatment is
the traditional one of protecting the germinating seedling against seed- and soilborne
fungi in the period immediately after planting (McGee, 1995). Chemical fungicides can
control plant diseases but they have bad
effects on human health, plants and animals, which is harmful to our environment.
Besides, using conventional seed treatment
with synthetic fungicides to kill pathogens
is a practice not allowed in organic production. Additionally, resistance by pathogens
to fungicides has rendered certain fungicides ineffective.
Worldwide
ecological
awareness
requires more natural foods and products,
which has influenced the improvement and
utilization of integrated pest management.
In this kind of control, alternative methods
are used to protect seeds to decrease the
use of chemical products. Moreover, recent
increases in the production and sale of organic
seed has heightened the scrutiny of organic
seed quality, and in particular brought
attention to concerns of seedborne disease
contamination. The number of alternative
crop production systems has increased in
the past decade in response to growing
concerns about agricultural concentration
and interest in a more ecological, farmbased agriculture. In these low-input systems, some non-chemical substances, such
as plant extracts, may be used successfully
as a contact fungicide seed treatment for
organic crops. As a rich source of bioactive
chemicals, plants may provide potential
alternatives to be used as pathogen-control
agents.
Hamburger and Hostettmann (1991)
report that the total number of plant chemicals may exceed 400,000 and of this, more
than 10,000 are secondary metabolites whose
major role in plants is defensive in nature.
Thus, plant-based secondary metabolites
that have a defensive role may be exploited
for the management of diseases and pests.
However, most species of higher plants
have never been surveyed. Their chemical
or biologically active constituents that
have the potential to be used as new sources
of commercially valuable pesticides remain
to be discovered. This is due mainly to the
lack of information on the screening/evaluation of diverse plants for their antifungal
potential (Satish et al., 2007). Nevertheless, several higher plants and their constituents have shown success in plant
disease control and have proved to be harmless and non-phytotoxic, unlike chemical
fungicides.
Essential Oils to Reduce
Seedborne Fungi
Plant extracts have played a significant role
in reducing the incidence of seedborne
pathogens and in the improvement of seed
quality and the emergence of plant seeds
in the field (Hasan et al., 2005). In recent
years, much attention has been paid to
essential oils, a group of plant-derived compounds, for seed treatment to protect them
Use of Plant Extracts as Natural Fungicides
against seedborne fungi (Sisterna and Dal
Bello, 2007).
The essential oils arise from a secondary metabolism of the plant, normally formed
in special cells or groups of cells as glandular hairs, found on many leaves and stems.
Oils occur as a globule or globules in the
cell and may also be secreted from cells lining the schizogenous ducts or canals. Plant
volatile oils are generally isolated from nonwoody plant material by several methods,
usually distillation, and are a variable mixture of principally terpenoids, specifically
monoterpenes [C10] and sesquiterpenes
[C15], although diterpenes [C20] may also be
present. A variety of other molecules can
also occur, such as aliphatic hydrocarbons,
acids, alcohols, aldehydes, acyclic esters or
lactones and, exceptionally, nitrogen- and
sulphur-containing compounds, coumarins
and homologues of phenylpropanoids (Dorman and Deans, 2000). Faleiro et al. (2003)
have shown that the antimicrobial action is
determined by more than one component.
In such cases, the major component is responsible not only for the antimicrobial activity,
but also the synergistic effect that may take
place. The mixtures are extremely complex
and vary with environmental and genetic factors (Asplund, 1968; Cabo et al., 1986; Arras,
1988; Bhaskara et al., 1998; Vanneste et al.,
2002). Moreover, the composition of essential
oils from a particular species of plant can differ between harvesting seasons and between
geographical sources (Di Pasqua et al., 2005;
Di Pasqua, 2006).
Major active compounds from essential
oils are known for their broad-spectrum
antifungal activity against both human and
plant pathogens. These constituents can
either affect the physiological functions of
microorganisms or cause structural changes
of hyphae and spores (Arras et al., 1993;
Zambonelli et al., 2004; Kishore et al., 2007),
and different fungi appear to react differently
to these components (Szczerbanik et al., 2007).
The antifungal essential oils reduce hyphal
growth and also induce lysis and cytoplasmic evacuation in fungi. Growth inhibition
by essential oils often involves induction of
changes in cell wall composition (Ghfir
et al., 1997), plasma membrane disruption,
53
mitochondrial structure disorganization (de
Billerbeck et al., 2001) and interference
with enzymatic reactions of the mitochondrial membrane, such as respiratory electron
transport, proton transport and coupled phosphorylation steps (Knobloch et al., 1989).
The active components vary between
oils. For example, the main component is
l-carvone in spearmint (Mentha spicata L.),
terpinen-4-ol in tea tree (Melaleuca alternifolia (Maiden. & Betche.) Cheel.) oil and
α-terpineol in pine (Pinus spp.) (Knobloch
et al., 1989). The essential oils of Cinnamomum zeylanicum Blume (cinnamon) and
Syzygium aromaticum (L.) Merr. & Perry
(syn. Eugenia cariophyllata Thunb.), consisting of cinnamaldehyde and eugenol,
respectively, as major components (Paranagama, 1991), are known to be potent antifungal materials (Beg and Ahmad, 2002;
Ranasinghe et al., 2002). Citral and geraniol
are the major components in essential oils
of Cymbopogon citratus (DC.) Stapf (lemongrass) and C. martinii (Roxb.) Stapf var.
motia (palmarosa), respectively, which are
antifungal compounds (Paranagama et al.,
2003; Velluti et al., 2004). Thymol was
identified as the active ingredient of Ocimum gratissimum L. (wild basil) and has
been found to suppress fungal growth
(Adekunle and Uma, 2005). Linalool is a
major component in the essential oil of Thymus mastichina L. subsp. mastichina, with
antimicrobial activity (Faleiro et al., 2003),
and both limonene and linalool are the
minor components in the essential oils
derived from different plants. The majority
of these essential oils and their components
have proved valuable in protection against
postharvest fungal diseases which cause
build-up of toxic fungal metabolites in
stored foods (Kishore et al., 2007). Therefore, essential oils might substitute agrochemicals or contribute to the development
of new agents to inhibit both fungal growth
and the production of mycotoxins affecting
grain and seed crops.
This chapter discusses the current status of plant extracts and the potential use of
essential oils as natural antifungal agents to
control the main seedborne pathogens and
spoilage fungi.
54
G. Dal Bello and M. Sisterna
Symptoms on Seeds
Caused by Fungi
Seedborne mycoflora comprise a large
number of saprobes and pathogenic fungal
species. Pathogenic fungi grown on seeds
can cause heavy damage and reduce yields
of seed, both quantitatively and qualitatively
(Neergaard, 1979). Other fungi, including
saprophytes and very weak parasites (Sisterna and Lori, 2005), may lower the quality
of seeds by causing discoloration, which
may seriously depreciate the commercial
value of seeds, particularly of grain when
graded for consumption.
Disease and disorder
The following types of disease and disorder
are encountered, often in combination
(Neergaard, 1979):
Seed abortion
The most prominent examples of fungi producing abortion are the smut fungi, which
infect cereals and grasses systemically, and
the ergot fungi. The floral parts of the hosts
are replaced by the fructifications of the
parasites. Other examples are different species of Fusarium (in wheat, maize and rice);
Ascochyta rabiei in chickpea may kill the
young seeds; Drechslera verticillata causes
death of seed primordia in brome grass and
in wheat.
Shrunken seeds, reduced in size
Examples of more or less heavy reduction of
seed size are: Alternaria brassicicola and
Phoma lingam in crucifers, Septoria linicola in flax, D. teres in barley, F. graminearum
and S. nodorum in wheat.
B. maydis and B. oryzae in cereals; Colletotrichum graminicola, Diaporthe phaseolorum (Plate 2) and Fusarium spp. in
soybean (Plate 3); Botrytis allii on onion
(Plate 4); Verticillium dahliae on spinach
(Plate 5) and B. cinerea in the seeds of many
hosts, including forest trees.
Sclerotization and stromatization
Transformation of floral organs or seed into
sclerotia or stromata is an important disease
condition in certain categories of fungi and
host. Ergots produced by Claviceps purpurea and other species of Claviceps in cereals
and grasses exemplify sclerotia of this type.
Another example is Phomopsis viterbensis
in chestnut, Pleospora herbarum in spinach
(Plate 6) and Ciboria spp. in the seeds of forest trees and grasses.
Seed necroses
Many seed-rotting fungi produce superficial
necroses in the seed; other fungi never penetrate deeply into the tissues, most seedborne fungi usually not beyond the protective
layers, the seed coat or pericarp. Anthracnose fungi, Colletotrichum spp. as well as
Ascochyta spp., often penetrate into the
fleshy cotyledons, producing conspicuous
necrotic lesions in the seeds of bean, soybean, pea, cowpea and other hosts.
Seed discoloration
Discoloration of seeds is a very important
degrading factor, both for consumption
(grain) or for industrial purposes (oil seed).
It may be a general indication of poor quality (Plates 7 and 8). Well-known examples
are the effects of A. pisi in pea; C. lindemuthianum in bean; B. sorokiniana in
wheat, B. oryzae in rice, Cercospora kikuchii
(Plate 9) in soybean, etc.
Seed rot
Reduction or elimination of germination
capacity, lowered viability
Many seedborne fungi produce seed rot either
in the crop or during germination. Examples
are F. avenaceum, F. graminearum (Plate 1),
F. moniliforme, Bipolaris sorokiniana,
Obviously, necroses or more deeply penetrating rots in seeds reduce the viability of
the seeds, their longevity in storage and their
emergence in the field.
Use of Plant Extracts as Natural Fungicides
Physiological alterations or effects in seed
Metabolic products of seedborne microorganisms may affect the seed itself or may
have other, sometimes serious consequences
such as toxicity to animals and humans
(Aspergillus spp., Penicillium spp., Fusarium spp.).
Moreover, seed fungi are classified as
field and storage fungi (Christensen and Kaufmann, 1965). Genera such as Alternaria,
Cladosporium (Plate 10), Fusarium and
Bipolaris invade seeds as they are developing on the plants in the field or after they
have matured, but before they are harvested,
and for this reason, they have been designated ‘field fungi’. These fungi require moisture content in equilibrium with a relative
humidity of more than 90% to grow and usually do not continue to grow in grains after
harvest, since grains and seeds are stored
with moisture contents below those required
by the field fungi.
The storage fungi consist mainly of
several species of Aspergillus. Species of
Penicillium are encountered at times, usually in lots of grain stored at low temperatures and with moisture contents above
16%. The storage fungi do not invade grains
to any appreciable degree or extent before
harvest.
Fungicidal Effects of Plant Extracts
Against Seed Fungi
Numerous studies have described the use of
botanicals with a view to exploiting their
potential as natural fungicides against seedborne fungi. The following section discusses
this alternative method, with particular
emphasis on the main seedborne fungal
pathogens.
Alternaria
A. padwickii, an important seedborne pathogen of rice (Oryza sativa L.), was inhibited by
aqueous extracts of Strychnos nux-vomica L.
(strychnine tree), garlic (Allium sativum L.)
55
bulbs, ginger (Zingiber officinale Roscoe)
rhizomes, basil (O. basilicum L.) leaves, and
fruits of Azadirachta indica A. Juss. (neem)
(Shetty et al., 1989).
Positive effects have been recorded on
the same fungus with essential oils of C. citratus, O. gratissimum L. and Thymus vulgaris L. (thyme) (Nguefack et al., 2004). The
researchers investigated the ability to control seedborne infection and seed–seedling
transmission in naturally infected seeds.
The essential oils increased the germination
capacity of the treated seeds.
Bipolaris
Hasan et al. (2005) demonstrated that plant
extracts, namely Z. officinale, A. sativum,
A. cepa L. (onion), Adhatoda vasica Nees
(vasaka), Achyranthes aspera L. (devil’s
horsewhip), A. indica, Lawsonia alba Lamarck (henna), Cuscuta reflexa Roxb. (giant
dodder), Vinca rosea L. and Nigella sativa
L. (black cumin), significantly reduced seed
infection of wheat by B. sorokiniana (Triticum aestivum L.). Alcoholic extracts of
neem and garlic inhibited the presence of B.
sorokiniana completely, whereas the highest percentage of the fungus was recorded
from untreated seeds (control). Water extract
of all tested plants had the ability to control
seedborne fungi of wheat var. Kanchan,
which showed 100% inhibition of B. sorokiniana with the application of extracts from
Z. officinale, A. sativum, A. cepa, A. indica,
C. reflexa and N. sativa, whereas the highest
fungal incidence (11.67%) was observed on
untreated seed. After treatment with the
water extract of L. alba and A. aspera, only
4.84% and 7.16% incidence of the pathogen, respectively, was recorded. Seeds of
wheat treated with A. vasica and V. rosea
gave statistically identical results (5.83%
and 5.90% incidence of B. sorokiniana).
Alice and Rao (1986) reported good
results using plant extracts to control B.
oryzae on rice seeds which have high natural infection of the fungus. After soaking in
the filtrates of different extracts, A. sativum
and M. piperita (peppermint) reduced seed
56
G. Dal Bello and M. Sisterna
infection by 68%. In Bangladesh, use of
extracts of Polygonum hydropiper L. (waterpepper), A. cepa, A. sativum and A. indica
was demonstrated to be effective against B.
oryzae at higher concentrations. Among
them, neem and garlic were the most effective at 1:1 dilution and inhibited the occurrence of the pathogen by 91 and 83%,
respectively (Ahmed et al., 2002).
Neem and pungam (Pongamia pinnata
(L.) Pierre) oil-based emulsifiable concentrate (EC) formulations were evaluated for
their efficacy to inhibit the mycelial growth
of the fungus Helminthosporium oryzae
(syn. B. oryzae) causing grain discoloration
of rice under in vitro conditions. All three
formulations, namely neem oil 60 EC (acetic
acid), neem oil 60 EC (citric acid) and neem
oil + pungam oil 60 EC (citric acid), inhibited mycelial growth of the pathogen; they
were effective even after 9 months of storage. These formulations controlled the grain
discoloration on rice effectively (Rajappan
et al., 2001). The efficacy of essential oils such
as clove, ginger, lemongrass, basil, peppermint, anise (Pimpinella anisum L.) and cinnamon at different concentrations on growth
inhibition of B. oryzae was examined by
Palaoud (2006). Treatments with clove, anise,
ginger and cinnamon oils at 500 ppm provided the best results in controlling the fungus and, after storage for 4 months, seed
viability was as high as 97–98%. Also, the
extracts of C. citratus, O. gratissimum and T.
vulgaris applied to rice seeds infected with
B. oryzae controlled fungal growth and seedling transmission of the pathogen (Nguefack
et al., 2004).
Colletotrichum
Abdelmonem et al. (2001) screened oils of
M. piperita, T. capitatus (L.) Hoffmans. and
Link and Carum carvi L. (caraway) against
various seedborne fungi of soybean (Glycine
max (L.) Merr.) and lentil (Lens culinaris
Medik.) and found all plant extracts to be
highly effective in controlling C. dematium.
Among the fibre-producing species, a
study on garlic bulb extract reported a
fungicidal effect of the oil against numerous
seedborne fungal pathogens of white jute
(Corchorus capsularis L.), one of the most
important crops from Bangladesh, India and
China. The essential oil produced inhibition in both mycelial growth and spore germination of fungi, including C. corchori
(Ahmed and Shultana, 1984), which was
also strongly inhibited in in vitro tests by
using crude leaf extracts from Eupatorium
triplinerve Vehl. (yapana) (Rahman and
Junaid, 2008).
Several studies carried out in Burkina
Faso underlined the antifungal properties of
extracts from some Cymbopogon spp. against
C. graminicola, the causal agent of anthracnose on sorghum (Sorghum bicolor (L.)
Moench and pearl millet (Pennisetum glaucum (L.) R. Br.). Somda et al. (2007) demonstrated that the essential oil of C. citratus at
a concentration of 6% was effective in controlling seedborne infection and seed–
seedling transmission of C. graminicola
without affecting seedling development.
Similarly, the essential oils extracted from
C. giganteus (Hochst.) Chiov., C. nardus (L.)
Rendle and C. schoenanthus Spreng. reduced
sorghum seed infection by the pathogen significantly. The lowest rates of infected seeds
were recorded on seeds treated with 10 µl
and 15 µl of C. nardus oil/g seeds. These
doses were more efficient than chemical
control (Elisabeth et al., 2008).
Curvularia
In blackgram (Vigna mungo L.), essential
oils extracted from wood chips of cedar
(Cedrus deodara (Roxb. ex Lamb) G. Don) and
that from seeds of Trachyspermum ammi (L.)
Sprague ex Turrill (ajowan) exhibited absolute toxicity, inhibiting the mycelial growth
of C. ovoidea, storage fungi found on seeds
(Singh and Tripathi, 1999).
Parimelazhagan and Francis (1999)
reported reduction in the radial growth of C.
lunata associated with rice seeds when
treated with leaf extracts of Clerodendrum
viscosum Vent. (glory tree), which also
increased seed germination and root and
Use of Plant Extracts as Natural Fungicides
shoot lengths of rice. Considerable research
activity has occurred in the Asian-Pacific
region on the potential for plant extracts to
control seedborne fungi including maize.
The oils of cassia (C. cassia Blume) and clove
inhibited the growth of established seedborne infections of C. pallescens (Chatterjee,
1990).
Fusarium
The essential oils and their constituents have
been found effective as antifungal agents
against the main species of Fusarium. Among
several plant extracts, Sitara et al. (2008)
found that essential oils from seed of neem,
black cumin and asafoetida (Ferula asafoetida L.) showed fungicidal activity of varying
degree against F. oxysporum, F. moniliforme
(syn. F. verticillioides), F. nivale and F. semitectum. Of those oils, asafoetida oil at 0.1%
and 0.15% inhibited the growth of all test
fungi significantly. A variety of wild plants
from Mexico were evaluated against several
cereal seedborne fungi in in vitro tests
(Tequida-Meneses et al., 2002). Extracts from
leaves and stems of Larrea tridentata (Sessé
& Moc. ex DC.), Coville (creosote bush) and
Datura discolor Bernh. (desert thorn apple)
in methanol or ethanol inhibited the radial
growth of F. poae completely. Next to these
extracts, Proboscidea parviflora (Woot.)
Woot. & Standl. (double claw) also showed
good fungal inhibition (86.6%), followed by
Baccharis glutinosa Pers. (saltmarsh baccharis) (79.6%), compared to the alcoholic
controls (0% inhibition).
In legumes, soybean and lentil, carvone
(monoterpene compound), among other
tested compounds, manifested the highest
antimicrobial influence with complete inhibition to F. oxysporum. It showed broadspectra activity against all the tested isolates
of fungal strains at low concentrations. Peppermint, T. capitatus and caraway oils also
demonstrated a high control effect against
Fusarium sp. (Abdelmonem et al., 2001).
Also, the antifungal activity of the essential
oils of thyme, clove, peppermint, soybean
and peanut (Arachis hypogaea L.) were
57
tested against F. oxysporum and F. equiseti in
vitro on cowpea (V. unguiculata (L.) Walp.)
(Kritzinger et al., 2002). Likewise, plant leaf
extracts (crude and aqueous) of basil, bitter
leaf (Vernonia amygdalina Del.), neem and
pawpaw (Carica papaya L.) reduced the incidence of F. moniliforme significantly and
increased seed germination and seedling
emergence of African yam bean (Sphenostylis stenocarpa (Hochst ex. A. Rich) Harms)
when compared with the untreated controls
(Nwachukwu and Umechuruba, 2001).
Regarding cereals, several natural plant
compounds have been identified as having
antifungal activity against seedborne fungi.
The essential oils of C. citratus, O. gratissimum and T. vulgaris have proved valuable
in protection against the seedborne fungus,
F. moniliforme in rice. This study evaluated
the ability to control seedborne infection
and seed–seedling transmission in naturally
infected seeds (Nguefack et al., 2004). The
extracts applied controlled seed infection
and seedling transmission of the pathogen
and increased the germination capacity of
the treated seeds. In the field, as a result of
extracts seed treatment as compared to the
non-treated control, reduction of disease
incidence and important increases in yield
were recorded. After rice seeds inoculated
with F. moniliforme were soaked in seven
plant essential oils at ten different concentrations, anise, ginger, clove and cinnamon
oils at 500 ppm provided the best results in
controlling the fungus. The percentage of
seed germination and the number of normal
seedlings was significantly high when compared with the control. Anise and clove also
showed the highest seedling dry weight
(Palaoud, 2006).
In another study on wheat, ten plant
extracts were tested for their efficacy in vitro
against seedborne fungi; alcoholic extract
of neem and garlic controlled the infection
of Fusarium sp. completely. Good results of
these treatments contributed to increased
seed germination (Hasan et al., 2005). Furthermore, botanicals from male fern (Dryopteris filix mas (L.) Scott.) suppressed
completely the population of F. oxysporum
in the seed mycoflora of wheat (Rake et al.,
1989). Putative mycotoxicogenic fungi, such
58
G. Dal Bello and M. Sisterna
as F. moniliforme, were partially or totally
sensitive to different essential oils extracted
from 12 medicinal plants (Soliman and
Badeaa, 2002). Results indicated that oils
of thyme, cinnamon and anise (< or =
500 ppm), marigold (Calendula officinalis
L.) and caraway (< or = 2000 ppm), spearmint and basil (3000 ppm) inhibited this
fungus completely.
Bioassays using a poisoning technique
were carried out with C. citratus, O. gratissimum and T. vulgaris for the control of
seedborne fungi infecting maize (Zea mays
L.) seeds. The results disclosed the fungicidal properties of theses oils against F. verticillioides. These natural products control
the seedborne inoculum of the pathogen by
90% to 100%. Field trials conducted in the
humid forest and the warm savannah zones
of Cameroon have shown that these products are potential seed treatments that could
be used as substitutes for synthetic fungicides, which are usually unaffordable to
resource-limited farmers (Tagne et al., 2008).
Seed treatment with cinnamon, palmarosa
and lemongrass oils at 500 mg/kg showed
antimycotoxigenic ability against fumonisin B1 accumulation by isolates of F. verticillioides and F. proliferatum (Marín et al.,
2003). Furthermore, different effects of oregano (Origanum vulgare L.) and herb Louisa
(Aloysia triphylla (L’Herit) Britton) essential oils were observed on F. verticillioides
M 7075 fumonisin B1 production in corn
grain in Argentina (López et al., 2004). As
alternative preharvest natural fungicides,
Velutti et al. (2004) showed the antimycotoxigenic activity of the essential oils against
F. graminearum on corn infested seed. The
effect of oregano, cinnamon, lemongrass,
clove and palmarosa on growth rate, zearalenone (ZEA) and deoxynivalenol (DON) production was assessed at two concentrations
(500 and 1000 mg/kg), at different water
activity and temperature levels. DON production in general was inhibited by all
essential oils at 30°C and, although palmarosa and clove were the only essential oils
with statistically significant inhibitory effect
on ZEA production, an inhibitory trend was
observed when cinnamon and oregano oils
were added to maize grain. Studies on the
efficacy of indigenous plant extracts against
seedborne infection of F. moniliforme on
maize demonstrated that aqueous extracts of
leaves of O. gratissimum, Acalypha ciliata
Forssk., V. amygdalina, M. indica L. (mango
tree) and A. indica had significant inhibitory growth effects on the fungal pathogen.
A. ciliata extract was more effective than
other plant extracts and compared favourably with benomyl in the control of the
pathogen (Owolade et al., 2000). Furthermore, to determine whether essential oils
can be used as a contact fungicide seed treatment for organic corn, the essential oils of
18 plants were screened for their fungicidal
properties. Five oils, cinnamon, clove, O.
minutiflorum O. Schwarz and P.H. Davis,
savoury (Satureja montana L.), and thyme,
controlled Fusarium completely in vitro.
The minimum inhibitory concentration
(MIC) was 800 µl/l and seedlings presented
no phytotoxicity symptoms in the germination test at rates up to 64 µl/kg active ingredient (MIC × 20). Field emergence of inbred
and hybrid seeds treated with the essential
oils were significantly lower than seeds
treated with the commercial fungicides,
Maxim XL {fludioxonil [4-(2,2-difluoro-1,3benzodioxol-4-yl)-1H-pyrrole-3-carbonitrile]
21.4%, mefenoxam [(R)-2-[(2,6-dimethylphenyl) methoxyacetylamino] propionic acid
methyl ester] 8.4%}, which is a conventional fungicide, and Natural 2 (proprietary
ingredients), which is an organic fungicide,
but were not different from the organic fungicide, Yield Shield (Bacillus pumilus GB34
0.28%) or an untreated control (Christian
and Goggi, 2008).
Bioactivity of different plant extracts on
F. thapsinum pathogen of sorghum was
evaluated on seeds contaminated with the
fungi. Cinnamon, clove, epazote (Teloxys
ambrosioides (L.) Weber), oregano and
thyme, singly and in combination, as well as
the essential oils of Mentha sp. and rue (Ruta
chalepensis L.) and the combination of clove
with cinnamon, had a fungicidal effect. Nevertheless, only thyme did not affect either
seed germination or sorghum seedling height.
The rest of the oils were phytotoxic (MontesBelmont and Flores Monctezuma, 2001).
Additionally, plant extracts were also tested
Use of Plant Extracts as Natural Fungicides
on naturally infected sorghum seeds for
controlling F. moniliforme. More than 50%
of the growth of this fungus was reduced by
C. citratus essential oil on seeds, whereas
Eucalyptus camaldulensis Dehnh. (Eucalyptus) essential oil was less efficient, even at
high concentrations (Somda et al., 2007).
Elisabeth et al. (2008) investigated the efficacy
of essential oils extracted from C. schoenanthus, C. nardus and C. giganteus in controlling Fusarium sp. on seeds of sorghum
and pearl millet. The results indicated that
all the essential oils reduced seed contamination of both cereals significantly. The lowest rates of infected seeds were recorded on
seeds treated with 10 µl and/or 15 µl of
essential oil/g seeds. Most of the time, these
doses were as efficient as the chemical control and oil of C. giganteus used at 15 µl/g
seeds eliminated pearl millet seed infection
by Fusarium completely.
In another experiment, de Souza et al.
(2003) analysed the mycoflora and physiological quality of cotton (Gossypium hirsutum
L.) seeds treated with chemical fungicides
and aroeira (Astronium urundeuva L.) extract.
Pure extract did not control the fungal population but, when mixed with the fungicides, captan, benomyl and tolylfluanid,
they showed reduction in the incidence of
Fusarium sp.
From Leguminosae members, leaf extracts
of Delonix regia (Bojer) Raf., flamboyant tree,
Pongamia glabra Vent. (Karanja) and Acacia
nilotica (L.) Willd. ex Delile (gum arabic tree)
significantly inhibited spore germination,
mycelial growth and spore production of F.
solani from sunflower (Helianthus annuus L.)
seeds (Thiribhuvanamala and Narasimhan,
1998). The same pathogen could be controlled
using crude leaf extracts of A. indica and
O. gratissimum to protect egusi melon (Cucumeropsis mannii Naudin) seed. After 6
months incubation, all the seeds treated
with leaf extracts showed no Fusarium
infection (Adekunle and Uma, 2005).
Efficacy of some plant extracts in controlling seedborne Fusarium infections of
mustard (B. nigra (L.) W.D.J. Koch) was
evaluated. It was found that garlic and neem
extracts were the most effective in controlling the pathogen among the plant extracts
59
studied. The effectiveness of garlic extract
was comparable to the fungicide, Rovral
(Latif et al., 2006).
Macrophomina
Several natural plant compounds have been
identified as having antifungal activity
against M. phaseolina. The work of Ahmed
and Shultana (1984) reported that garlic oil
produced inhibition in both mycelial growth
and spore germination of M. phaseolina, an
important seedborne fungal pathogen of
jute. In sunflower seeds, the leaf extracts of
the flamboyant tree, karanja and gum arabic
tree significantly inhibited the germination
of fungal spores, mycelial growth and spore
production as well (Thiribhuvanamala and
Narasimhan, 1998).
In vitro experiments conducted by
Dwivedi and Singh (1999) confirmed the
fungitoxicity of some higher plant extracts
against the mycelial growth of M. phaseolina. Among the plant products, the essential
oils of T. ammi exhibited absolute fungicidal effect at an MIC of 200 ppm.
Studies of Abdelmonem et al. (2001)
also showed the inhibitory effect of the
essential oils of M. piperita, T. capitatus
and C. carvi against M. phaseolina associated with the seeds of soybean and lentil.
Furthermore, when tested in infected cowpea seeds, A. indica extract was found to
inhibit the incidence of the pathogen. After
naturally infected seeds were immersed in a
suspension containing neem tree oil at a
concentration of 0.5% for 16 h, the infection incidence decreased to 50% in relation
to controls using only water (Mello et al.,
2005).
Aspergillus and Penicillium
Putative mycotoxicogenic fungi of wheat
grains were partially or completely sensitive to different essential oils extracted from
12 medicinal plants (Soliman and Badeaa,
2002). They were tested for inhibitory
activity against A. flavus, A. parasiticus and
60
G. Dal Bello and M. Sisterna
A. ochraceus. Results indicated that oils of
thyme, cinnamon (< or = 500 ppm), marigold (< or = 2000 ppm), spearmint and basil
(3000 ppm) inhibited all the tested fungi
completely. Caraway was inhibitory at
2000 ppm against A. flavus and A. parasiticus and at 3000 ppm against A. ochraceaus.
Also, the three species were suppressed by
anise at < or = 500 ppm. An in vitro initial
screening of a range of several spice hydrosols on inhibition of mycelial growth of A.
parasiticus revealed that hydrosols of anise,
cumin (Cuminum cyminum L.), fennel
(Foeniculum vulgare Mill.), Mentha sp.,
oregano, savoury and thyme caused a stronger inhibitory effect on mycelial growth
(Özcan, 2005).
When essential oil from oregano was
applied as a fumigant against the mycelia
and spores of A. flavus, A. niger and A.
ochraceus on wheat, the oil vapour exhibited
a fungicidal effect and a significant reduction
in the per cent of infested grain was observed
(Paster et al., 1995). Plant extracts of Z. officinale, bulbs of A. sativum and A. cepa, leaves
of A. vasica, L. alba, A. indica, A. aspera,
stem of C. reflexa, root of V. rosea and seeds
of N. sativa were tested for their efficacy in
vitro against Aspergillus sp. and Penicillium
sp. in wheat. All the plant extracts reduced
the incidence of seedborne fungi significantly and increased seed germination, the
number of healthy seedlings and the vigour
index. Neem and garlic extracts controlled
the intensity of the fungi completely (Hasan
et al., 2005).
A natural fungicide against aflatoxigenic fungi to protect stored rice using the
essential oil of C. citratus was developed by
Paranagama et al. (2003). Lemongrass oil
was tested against A. flavus and the test oil
was fungistatic and fungicidal against the
test pathogen at 0.6 and 1.0 mg/ml, respectively. Aflatoxin production was completely
inhibited at 0.1 mg/ml. The results obtained
from the thin layer chromatographic bioassay and gas chromatography indicated citral
a and b as the fungicidal constituents in
lemongrass oil. During the fumigant toxicity
assay of lemongrass oil, the sporulation and
the mycelial growth of the test pathogen
were inhibited at concentrations of 2.80 and
3.46 mg/ml, respectively. Therefore, lemongrass oil could be used to manage aflatoxin
formation and fungal growth of A. flavus in
stored rice. Besides, the essential oil of lemongrass inhibited growth of moulds like A.
flavus, A. fumigatus and P. chrysogenum of
maize and cowpea grains. Within a storage
period of 10 days, seeds of maize and cowpea treated with lemongrass powder and
essential oil showed no physical deterioration. Off-colour, off-odour and mouldiness,
however, characterized untreated control
seeds (Adegoke and Odelusola, 1996).
Another assay on A. flavus determined optimal levels of dosages of 11 plant essential
oils for maize kernel protection, effects of
combinations and residual effects (MontesBelmont and Carvajal, 1998).
Bankole (1997) showed that essential
oils from A. indica and Morinda lucida
Benth. (brimstone tree) inhibited the growth
of a toxigenic A. flavus and reduced aflatoxin
B1 synthesis significantly in inoculated
maize grains. Studies in experimental grain
bins have demonstrated that soybean oil
alone also reduces infection by storage fungi
(White and Toman, 1994). After 12 months,
kernel infection by Penicillium spp. and
Aspergillus spp. was 83% and 63.7%, respectively, in untreated corn, compared to 60%
and 46.2%, in soybean oil-treated corn at
200 ppm (McGee, 1989). Essential oils from
aromatic plants such as cinnamon, clove,
oregano, savoury and thyme inhibited the
growth of the corn pathogen Penicillium sp.
completely in vitro. The MIC of the essential oils in the laboratory was 800 ppm. The
growing seedlings were not affected and no
phytotoxicity symptoms were seen at rates
up to 16,000 ppm concentration of the oils
(Goggi et al., 2008). A previous work was
undertaken by Chatterjee (1990) to screen
some essential oils for their inhibitory activity against fungal infection and mycelial
growth in postharvest maize grains during
storage. It was observed that the oils of Cassia sp. clove (30 ml/g grain and above), star
anise (Illicium verum Hooker fil.) (40 ml/g
grain and above), Geranium sp. (30 ml/g grain
and above) and basil (50 ml/g grain) inhibited
the in vivo mycelial growth of established
seedborne infections of A. flavus, as well as
Use of Plant Extracts as Natural Fungicides
preventing infection following inoculation
with A. flavus, A. glaucus, A. niger and A.
sydowi. These oils also preserved the grain
from natural A. flavus infection during the
experimental period. Christian and Goggi
(2008) studied whether essential oils could
be used as a contact fungicide seed treatment for organic corn. In vitro, the essential
oils of cinnamon, clove, oregano, savoury
and thyme controlled Penicillium completely. Soybean oil, applied at a rate used
to suppress grain dust, reduced storage
fungi growth in maize and soybeans in field
storage bins. After 12 months, soybean seed
infection by Penicillium spp. and Aspergillus spp. was 45.7% and 39.2%, respectively,
in untreated seeds, 17.7% and 8.2% in soybean oil-treated seeds and 1.7% and 2% in
soybean oil + thiabendazole-treated seeds
(McGee, 1989; White and Toman, 1994).
Also, soybean oil demonstrated its effectiveness in decreasing by 50% the levels of
seed infection and physiological ageing by
the storage fungus, A. ruber, on garden pea
seeds (Pisum sativum L.) (Hall and Harman,
1991). Peppermint, thyme and clove oils
were tested in vivo against A. flavus, A. niger
and P. chrysogenum on different seed cultivars of cowpea. Antifungal activity was
observed for the three oils, depending on
cultivar and concentrations (Kritzinger et al.,
2002). In blackgram, essential oils extracted
from wood chips of cedar and that from the
seeds of ajowan exhibited absolute toxicity,
inhibiting the mycelial growth of A. niger on
storage seeds (Singh and Tripathi, 1999).
Major seedborne fungi associated with
African yam bean like A. niger and A. flavus
could be controlled by using leaf extracts
(crude and aqueous) of basil, bitter leaf,
neem and pawpaw. All the plants’ leaf
extracts reduced significantly the incidence
of fungi tested and increased seed germination and seedling emergence when compared
with the untreated controls. The crude
extracts were most effective, mainly neem,
which gave complete control of A. niger and
A. flavus. In addition, seed germination was
enhanced by this extract and reached nearly
90% (Nwachukwu and Umechuruba, 2001).
A. flavus is also found infesting seeds of
guar, Cyamopsis tetragonoloba (L.) Taub., a
61
native plant of India, whose main commercial value is due to its seed gum (galactomannan gum). In this case, A. flavus was
reduced by cumin oil extracted from seeds
(Dwivedi et al., 1991). Studies carried out
have shown that cumin has powerful antimicrobial properties against diverse species
of bacteria and fungi. The chemical studies
indicated that the greater part of this antimicrobial activity might be attributed to the
cuminaldehyde [p-isopropil benzaldehyde]
that is present in the dried fruit of this plant
(De et al., 2003).
Another study (de Souza et al., 2003)
investigated the mycoflora and physiological quality of cotton seeds treated with
chemical fungicides and aroeira extract.
Pure extract did not control the fungal population but, mixed with the fungicides, captan, benomyl and tolylfluanid, it showed
reduction in the incidence of Aspergillus
sp. Garlic extract was also found to be effective in removal of the seedborne pathogens
of mustard, including species of Aspergillus
and Penicillium (Latif et al., 2006).
Fungi of the genera Aspergillus and Penicillium are widely distributed storage fungi
of egusi melon seeds, causing seed discoloration, decreased nutritive value, increase in
free fatty acid and peroxide values, decreased
seed germination and producing a number
of toxic metabolites, including aflatoxin. Four
mould species, A. flavus, A. niger, A. tamarii and P. citrinum, were inoculated on to
shelled melon seeds. The essential oil of C.
citratus at 0.1 and 0.25 ml/100 g seeds
reduced deterioration and aflatoxin production significantly in shelled seeds inoculated with A. flavus. At higher dosages (0.5
and 1.0 ml/100 g seeds), the essential oil
prevented aflatoxin production completely.
After 6 months in farmers’ stores, unshelled
melon seeds treated with 0.5 ml/100 g seeds
of essential oil had a significantly lower
proportion of visibly diseased seeds and
Aspergillus spp. infestation levels and significantly higher seed germination compared to the untreated seeds. The efficacy of
the essential oil in preserving the quality of
melon seeds in stores was statistically on a
par with that of fungicide (iprodione) treatment (Bankole et al., 2005).
62
G. Dal Bello and M. Sisterna
Conclusions
While modern agricultural practices have
resulted in higher and more stable yields,
they have also weakened the natural balance between pests and their antagonists
and have reduced soil fertility and health.
Harmful chemicals threaten both the environment and human health alike. The benefits of pesticides, in terms of reduced crop
losses, are often overestimated because the
viability of alternative pest management
approaches is not fully understood. Conversely, the costs of relying predominantly
on synthetic pesticides in pest control, in
terms of health, environment, agroecology
and trade, are also not known completely
and consequently are often underestimated
(SP-IPM, 2008). Integrated pest management (IPM) has emerged as a way towards
maintaining or increasing agricultural productivity without over-reliance on synthetic
chemical pesticides, emphasizes the growth
of a healthy crop with the least possible disruption of agroecosystems and encourages
natural pest control mechanisms (FAO, 2002).
In this context, development of simple and
eco-friendly seedborne disease management
methods is necessary to improve the quality
of seed in general and farmers’ saved-seed
in particular (Elisabeth et al., 2008).
Plant-derived compounds as crop protectants represent a vast and rapidly progressing resource. Botanical fungicides are
best suited for use in industrialized countries
when strict enforcement of pesticide regulations is impractical, or in the case of organic
production. However, they can play a much
greater role in protecting crops in developing
countries, where human pesticide poisonings
are most prevalent. Among the plant products,
essential oils especially are a very attractive
method of controlling plant diseases. Essential
oils and their components are gaining increasing interest because of their relatively safe status, their wide acceptance by consumers and
their exploitation for potential multi-purpose
use. Besides, the problem of developing
resistant strains of fungi may be solved by
the use of essential oils of higher plants as
fumigants in the management of fungal
pathogens because of synergism between
different components of the oils (Varma and
Dubey, 1999; Dubey et al., 2008).
In recent years, tremendous strides have
been made in advancing the study of the
natural control of plant pathogens, particularly seedborne fungi. As is shown in this
chapter, plant metabolites and plant-based
fungicides appear to be one of the better alternatives, as they are known to have minimal
environmental impact and danger to consumers in contrast to synthetic pesticides. Despite
the potential of these naturally occurring biochemicals as biorational fungicides, their
practical development and implementation
will require more detailed studies. Efforts
should be made to search for indigenous
plants as a source of antifungal compounds
and to bioprospect the antifungal properties
of these plant products, especially essential
oils, towards seed fungi. Field trials are
required to assess the practical applicability
of botanical pesticides, together with bulk
production, extensive usage of active compounds and interaction with other IPM
components. Biosafety studies should be
conducted to ascertain their toxicity to
humans, animals and crop plants. Additional
screenings might be focused on the quality
assurance of botanicals and its regulation.
While it is unlikely that biopesticides
will replace chemical pesticides completely
in the foreseeable future, we can expect that
there will be some decline in the use of chemicals, particularly in developed countries.
Exploitation of naturally available chemicals
from plants, which retard the reproduction of
undesirable microorganisms, would be a
more realistic and ecologically sound method
for plant protection and will have a prominent role in the development of future
commercial pesticides for crop protection
strategies, with special reference to the management of plant diseases (Varma and Dubey,
1999; Gottlieb et al., 2002). The prospect of
botanical products as fungicides includes
plant compounds with broad-spectrum activity to provide protection against a range of
pathogenic fungi that attack the plant at the
same or subsequent growth stages following
their application. Furthermore, essential oils
are made up of many components that may
have synergistic effects; it may therefore be
Use of Plant Extracts as Natural Fungicides
expected that blends of essential oils or oil
components will be produced to control a
wide range of fungal species (Szczerbanik
et al., 2007). In the coming years, we envisage a broader appreciation of the attributes
63
of alternative methods and expect to see
synergistic combinations of semi-chemicals
with other technologies that will enhance
the effectiveness and sustainability of integrated control.
References
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France. International Seed Testing Association, Zürich, Switzerland, p. 58.
Adegoke, G.O. and Odelusola, B.A. (1996) Storage of maize and cowpea and inhibition of microbial agents
of biodeterioration using the powder and essential oil of Cymbopogon citratus. International Biodeterioration and Biodegradation 37, 81–84.
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Part II
Disease Control Through Resistance
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6
Resistance to Septoria Leaf
Blotch in Wheat
María R. Simón
Cerealicultura, Facultad de Ciencias Agrarias y Forestales,
Universidad Nacional de La Plata, La Plata, Argentina
Abstract
Mycosphaerella graminicola (Fuckel) Schroeter, in Cohn, is the causal agent of Septoria leaf blotch, an
important disease in many wheat-producing areas of the world which causes significant yield losses.
Breeding for resistance is the most economical approach to control the disease. Advances in the genetics of resistance and genetic variation of the pathogen population, as well as the new tools for a more
efficient incorporation of resistance in breeding programmes, are discussed.
Introduction
Bread wheat (Triticum aestivum L.) is the
most widely grown and consumed food
crop in the world. It is the staple food of
nearly 35% of the world population and the
demand for wheat will grow faster than for
any other major crop (Rajaram, 1999). The
forecast global demand for wheat in the year
2020 varies between 840 (Rosegrant et al.,
1995) to 1050 Mt (Kronstad, 1998). To meet
this demand, global production will need to
increase by 1.6–2.6% annually from the
present production level of 620 Mt.
Wheat breeding is focused on developing widely adapted, disease-resistant genotypes with high yields that are stable across
a wide range of environments. Incorporating durable resistance is a priority since
breeding for stable yields without adequate
resistance against the major diseases would
be impossible (Rajaram, 1999).
Diseases of wheat, mostly caused by
fungal pathogens and a few by viruses and
bacteria, are important production constraints
in almost all wheat-growing environments
(Rajaram and van Ginkel, 1996; McIntosh,
1998). Globally important fungal diseases of
wheat caused by obligate parasites include
the three rusts (leaf rust, caused by Puccinia
triticina Eriks., yellow rust caused by P.
striiformis West f. sp. tritici Eriks. and stem
rust caused by P. graminis Pers. f. sp. tritici
Eriks & Henn); powdery mildew caused by
Blumeria graminis tritici (DC) Speer; asexual
form Oidium monilioides (Nees) Link; stinking smut (Tilletia caries (DC) Tul. and C. Tul.
and T. foetida (Wallr. Liro.); loose smut (Ustilago tritici (Pers.) Rostr.); U. nuda (J.L. Jensen)
Kellerm. and Swingle. Those caused by facultative parasites include leaf blotch, M.
graminicola (Fuckel) J. Schröt., in Cohn,
asexual form S. tritici Rob ex Desm.; glume
blotch (Phaeosphaeria nodorum, asexual
form Stagonospora nodorum blotch); spot
blotch (Cochliobolus sativus (Ito and Kuribashani) Drechs. ex Dastus, asexual form Bipolaris sorokiniana (Sacc.) Shoem.); tan spot,
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
69
70
M.R. Simón
Pyrenophora tritici-repentis (Died.) Drechs.,
asexual form Drechslera tritici-repentis (Died.)
Shoemaker; Alternaria spp. (belonging to the
A. infectoria species groups); scab, Fusarium
graminearum Schwabe, take all (Gaeumannomyces graminis (Sacc.) von Arx and
Olivier var. tritici Walker).
Leaf blotch causes important yield
losses in many countries. Yield reductions
range from 31 to 54% (Eyal et al., 1987), from
10 to 45% (Caldwell and Narvaez, 1960) and
even yield losses higher than 60% have been
reported (Shipton et al., 1971). Sanderson
(1972) proved the connection between the
two stages and the sexual (teleomorph) form
has been reported in several countries (Hunter
et al., 1999). The sexual stage in Argentina
was reported by Cordo et al. (1990).
Mycosphaerella graminicola is a hemibiotrophic pathogen; early infection is biotrophic, followed by a switch to necrophic
growth just prior to symptom expression. The
sexual stage is also known to play a role in the
disease cycle. It causes most of the initial
infection of winter wheat crops during autumn
in the UK (Shaw and Royle, 1989) and the
USA (Schuh, 1990). In Argentina, an increase
in ascospores at harvest time has been reported,
suggesting that the sexual stage may be
important to initiate the infection in the next
growing season. Following stem elongation,
infection of the upper leaves of a crop has been
thought to be entirely due to the asexual stage
of the fungus, in which pycnidia give rise to
splash-dispersed pycnidiospores, which are
splash-dispersed from infected basal tissue to
the upper leaves by raindrops. However, more
recent work has shown that upward movement of inoculum can occur in the absence of
splashy rainfall, being influenced by the position of developing leaves in relation to infected leaf layers (Lovell et al., 1997). Another
possible means of spread within a crop during summer is by airborne ascospores,
which may play a role more important than
previously recognized (Hunter et al., 1999).
Types of Resistance
Although several control methods, including
cultural practices and the use of fungicides,
may reduce the effect of S. tritici blotch,
genetic resistance is the most cost-effective
and environmentally safe technique to manage the disease.
Monogenic or oligogenic and polygenic
resistance coexist in the pathosystem T.
aestivum/M. graminicola. Monogenic or oligogenic resistance is generally near complete,
isolate specific, follows the ‘gene-for-gene’
mode of inheritance and has been found in
several genotypes (Rillo and Caldwell, 1966;
Rosielle and Brown, 1979; Lee and Gough,
1984; Somasco et al., 1996; Arraiano et al.,
2001; Brading et al., 2002; McCartney et al.,
2002). Polygenic resistance is generally partial
and isolate non-specific and is also present in
several genotypes (Jlibene et al., 1994; Simón
and Cordo, 1997, 1998; Brown et al., 2001;
Zhang et al., 2001; Chartrain et al., 2004b).
Partial resistance is expressed as a
reduced epidemic development and is supposed to be durable. Several components
contribute to the epidemic-retarding effect.
Parlevliet (1979) mentioned four partial
resistance components: infection frequency,
latent period, spore production and infection period. The earliest studies on this type
of resistance, previous to the mapping of
genes and QTLs, investigated the gene effects
conditioning these components.
Several of the components of partial
resistance to M. graminicola may be controlled by just a few genes (Jlibene et al.,
1994). Danon and Eyal (1990) determined
that additive effects for pycnidial coverage
were the major variance component, although
dominance effects were also significant.
Jlibene et al. (1994) found that general combining ability (GCA) effects accounted for
most of the variation of percentage pycnidial
coverage, although specific combining ability
(SCA) effects were detected in some crosses.
Simón and Cordo (1997, 1998) determined
that GCA was preponderant for incubation
period, latent period, pycnidial coverage and
spore production, although SCA was also
significant. Incubation period was inherited
independently of maturation period and pycnidial coverage. Those components that are
genetically different and independent could
be combined into the same genetic background by crossing (van Ginkel and Rajaram,
Resistance to Septoria Leaf Blotch
1999), increasing the level of durable resistance. Significant correlations were found
between pycnidia/cm2 and spore/ml, indicating the feasibility of selecting for a lower
pycnidial density in order to obtain a reduction in spore production (Simón and Cordo,
1998). Heritability tends to be only moderate
(Simón et al., 1998), but progress in breeding
for resistance may still be possible. Major
genes are interesting because of the high
level of resistance and thus an almost complete absence of symptoms in the host; partial resistance, however, is very important
due to its putative durability and its expression under a broad spectrum of isolates of
the pathogen. A few genes may be enough to
confer resistance that will hold up in farmers’ fields (Dubin and Rajaram, 1996).
Resistance conditioned by a single dominant gene was assigned to some cultivars as
Bulgaria 88 (Rillo and Caldwell, 1966), Oasis
(Shaner and Buechley, 1989), Veranopolis
(Wilson, 1979) and others. Later, genes were
located and it was found for example that Stb1
conditioned resistance in Bulgaria 88 and
Oasis, Stb2 in Veranopolis, etc. Some other
cultivars showed resistance conditioned by
several major genes as Kavkaz 4500 L.6.A.4.
(Jlibene et al., 1992) and the genes were identified (Stb6, Stb7, Stb10 and Stb12; Chartrain
et al., 2005a). Also, three major genes were
identified in the Portuguese line TE 9111 (Stb6,
Stb7 and Stb11; Chartrain et al., 2005b). Furthermore, commercially grown cultivars range
from moderately resistant to susceptible, indicating the presence of partial resistance. Chartrain et al. (2004b) found high partial resistance
levels in several wheat cultivars from Europe
and Mexico (Arina, Milan, Senat). Simón et al.
(2005a) also found high levels of partial resistance in some Argentinian cultivars effective
to several isolates (Klein Volcán, Klein
Dragón) in adult stage. Some germplasm as
the Portuguese line TE 9111 (Chartrain et al.,
2005b) also has been proved to carry several
major genes together with partial resistance.
71
investigated the chromosomal location of
resistance using substitution lines. Resistance was found to be located on chromosome 7D from a synthetic hexaploid wheat
(T. dicoccoides × T. tauschii) in seedling
and adult stage to some specific isolates
(Simón et al., 2001, 2005b). Also, resistance
was found in chromosomes 1B at the seedling stage and on 5D at the seedling and
adult stage of the T. aestivum cv. Cheyenne
(Simón et al., 2001, 2005b); on the 2B, 3A
and 3B of the T. aestivum cv. CappelleDesprez at the seedling stage and on 6D and
7D of T. spelta with some specific isolates
(Simón et al., 2001, 2005b).
During the past decade, several genes
(Table 6.1) and QTLs (Table 6.2) have been
located. Some of them have proved to be
effective to isolates from several regions in
the world. Simón et al. (2007) tagged, using
isolates from Argentina, a gene in the 7D
chromosome of Aegilops tauschii, which is
likely Stb5. This would indicate that the
presence of Stb5 ensures resistance against
some isolates from both Europe (Portugal,
The Netherlands) (Arraiano et al., 2001) and
South America (Argentina).
Breeding for Resistance
The incorporation of resistance to the pathogen has been slow for several reasons, among
them:
1. The high variability of the pathogen
population.
2. The lack of knowledge of the virulence
spectrum.
3. The lack of relationship in the expression
of resistance in seedling and adult stage.
4. The influence of heading date and plant
height on resistance and the difficulty in assessing real values in breeding programmes.
Variability of the pathogen population
Location of the Resistance
Studies on the location of resistance began
during the past decade. Some of them
The population of the pathogen has been
studied and a high variability has been
found. Variation in virulence patterns within
72
M.R. Simón
Table 6.1. Major genes conditioning resistance to Mycosphaerella graminicola identified in hexaploid
wheat.
Locus
Chromosomal
location
Stb1
Stb2
5BL
3BS
Stb3
Stb4
Stb5
Stb6
Stb7
Stb8
Stb9
Stb10
Stb11
Stb12
Stb13
Stb14
Stb15
6DS
7DS
7DS
3AS
4AL
7BL
2B
1D
1BS
4AL
7BL
3BS
6AS
Linked markers
Reference
Xbarc74, Xgwm335
Xgwn389, Xgwm533.1, Xbarc133, Xbarc75,
Xgwm493
Xgdm132
Xgwm44, RC3, Xgwm11, Xgwm437, Xgwm121
Xgwm44, RC3, Xgwm111, Xgwm437, Xgwm121
Xgwm369, Xwmc11
Xgwm160, Xwmc219, Xwmc313
Xgwm146, Xgwm577, Xgwm611
Adhikari et al., 2004c
Adhikari et al., 2004b
Xgwm848
Xbarc008, Xbarc137
Xwmc219, Xwmc313
Xwmc396-7B
Xwm632-3B
Xpsr563a, Xpsr904
Adhikari et al., 2004b
Adhikari et al., 2004a
Arraiano et al., 2001
Brading et al., 2002
McCartney et al., 2002
Adhikari et al., 2003
Chartrain, 2004
Chartrain et al., 2005a
Chartrain et al., 2005b
Chartrain et al., 2005a
Cowling et al., 2007
Brule Babel, 2007
Arraiano et al., 2007
Table 6.2. Quantitative trait loci (QTLs) conditioning resistance to Mycosphaerella graminicola in
hexaploid wheat.
Locus
Chromosomal location
Linked marker
Reference
QStb.risø-2B
QStb.risø-3A.1
QStb.risø-3A.2
QStb.risø-3B
QStb.risø-6B.2
QtStb.risø-7B
QStb.ipk-1D
QStb.ipk-2D
QStb.ipk-6B
QStb.ipk-3D
QStb.ipk-7B
2BL
3AS
3BL
3B
6B
7B
1D (seedlings)
2D (seedlings)
6B (seedlings)
3D (adult)
7B (adult)
Xwmc1575a-Xwmc175a
Xgwm369
Xwmc489-Xwmc505
M62/P38-373
Xwmc397-Xwmc341
M49/P38-229-M49/P11-229
Xmwg938a
Xcdo405a
Xksuh4b
Xbcd515
Xksud2a
Eriksen et al., 2003
Eriksen et al., 2003
Eriksen et al., 2003
Eriksen et al., 2003
Eriksen et al., 2003
Eriksen et al., 2003
Simón et al., 2004a
Simón et al., 2004a
Simón et al., 2004a
Simón et al., 2004a
Simón et al., 2004a
and between populations was shown by
assessing host response on a selected set of
cultivars, with little similarity between the
results obtained with various sets of differentials (Eyal et al., 1995). Evidence for specificity was also confirmed by several
researchers (Danon and Eyal, 1990; Kema
and van Silfhout, 1997; Simón et al., 2005a).
Non-specific resistance to a wide set of isolates was also found (Simón et al., 2005a).
During the past decades, the population has
been studied using molecular markers and a
high variability within populations has
been confirmed (Chen and McDonald, 1996;
Zhan et al., 2001, 2003; Cordo et al., 2007).
The sexual state might have an impact on
the virulence spectrum in regions where
pseudothecia were found and ascospore
dispersal coincided with the wheat growing
cycle (Shaw and Royle, 1989; Lovell et al.,
1997). No attempts to determine races have
been carried out.
Recently, the genome of the pathogen
was sequenced completely (Goodwin et al.,
Resistance to Septoria Leaf Blotch
2007). The essentially finished sequence contains 18 chromosomes from telomere to telomere, plus five fragments, which presumably
make up two additional chromosomes. A
comparative bioinformatics analysis of M.
graminicola with seven other sequenced fungal genomes revealed that it possessed fewer
enzymes than expected for degrading plant
cell walls. The frequency of transposable elements in the genome of the pathogen was
intermediate between those of other sequenced
fungi. Availability of the finished genome for
M. graminicola should aid research on this
organism greatly and will help in the understanding of its interaction with wheat.
Expression of resistance
in seedlings and adults
Resistance is sometimes expressed in seedlings, sometimes at adult stage and sometimes at both stages (Kema and van Silfhout,
1997). Some germplasm with resistance at
both stages have been found (Arama, 1996;
Somasco et al., 1996; Simón et al., 2005a).
Influence of heading date and
plant height on resistance
One complicating factor for the assessment
of resistance level has been the influence of
heading date and plant height on the expression of resistance. Several scientists have
reported an increased disease level in earlier heading or shorter cultivars (Eyal et al.,
1987; van Beuningen and Kohli, 1990; Camacho Casas et al., 1995; Chartrain et al., 2004a).
Baltazar et al. (1990) suggested a genetic
association between shortness and susceptibility, while Eyal (1981) and Rosielle and
Boyd (1985) assumed a genetic association
between earliness and susceptibility. Arama
et al. (1999), Simón et al. (2005a) and Arraiano
et al. (2006) reported no genetic association
among those traits. Simón et al. (2004b,
2005a) determined that there was no influence of heading date when cultivars were
evaluated at the same development stage
under similar weather conditions and found
73
that the relationship between those traits
was caused mainly by environmental and
epidemiological
factors.
Associations
between pycnidial coverage percentage and
days to heading were positive or negative,
depending on whether weather conditions
before the evaluations were more conducive
to the development of the disease in late or
early heading cultivars, respectively. Negative associations with plant height were
only present in the years where weather
conditions were less conducive to the development of the disease. Inconducive conditions and longer distances between leaves in
tall cultivars could have reduced the rainsplash dispersal of pycnidiospores, thus
causing this negative association, mainly
when the sexual form is not present.
In most cases, previous reported associations between heading date and resistance could be attributed to the fact that the
disease was scored at the same time but not
at the same growth stage, causing early maturing lines to be exposed to inoculum for a
longer period than later maturing leaves.
Simón et al. (2009 unpublished) mapped a
population derived from T. spelta 7D/Chinese Spring where QTLs conditioning resistance were found, but no genes for heading
date were present. Also, some QTLs for resistance were mapped in a Synthetic 6 × (T.
tauschii × Altar 84) × Opata 85 (Simón et al.,
2004a), which did not coincide with the
regions where QTLs for flowering time were
previously mapped. Eriksen et al. (2003)
located in a double haploid population originated from the cross of Savanah and Senat, a
QTL for increasing plant height linked to a
QTL for resistance. Although associations
could exist in some germplasm, pleiotropic
effects have not been detected and breeders
can select for S. tritici blotch resistance within
a range of heading dates and plant heights.
Resistance and Integrated
Management
It is necessary to consider that integrated
management can contribute to the durability
of resistance. Epidemiological advantages
can be derived by combining management
74
M.R. Simón
practices and through disease management
on a regional scale. Diversifying sources of
partial resistance, on a field or regional
basis, might slow pathogen adaptation. Populations of M. graminicola sampled from
mixtures of a susceptible and a partially
resistant wheat cultivar were all less fit than
populations derived from the same cultivars
grown in pure stand (Mundt et al., 2002).
Cultural practices such as adequate
tillage method, planting density and Nfertilization conditions, together with fungicide applications, are important to the
appropriated expression of resistance. The
planting of no-till wheat may increase
the level of Septoria leaf blotch because
increasing levels of crop residue on the soil
surface potentially increase primary inoculum of plant pathogens, mainly under continuous wheat production or wheat/soybean
sequences in the same year. Since the pathogen can survive in infested wheat residues
for about 2 years, a rotation where wheat is
planted in only 1 of 3 years is recommended.
Although there are contrasting results, several reports indicate that, under conducive
conditions for the development of the disease, an increase in N-fertilization causes a
slight increase in severity (Hayden et al.,
1994; Howard et al., 1994; Leitch and Jenkins, 1995; Simón et al., 2002, 2003).
Conclusions
Research on Septoria leaf blotch has
expanded greatly in the past decades. New
molecular tools enable the exploration of
biological issues associated with the pathogen, the host and the host–pathogen interaction. Several genes and QTLs have been
identified and mapped. The major challenge to wheat breeders and plant pathologists is the selection and development
of cultivars with durable resistance. To
achieve this goal, the incorporation of
marker-assisted selection into breeding
programmes will speed pyramiding several
genes or QTLs effective at different stages
of plant development into single wheat
cultivars to develop broad-spectrum and
durable resistance.
Management of cultivars should be
optimized to minimize the associations
between heading date, height and resistance, but selection of early and short lines
with high levels of quantitative resistance is
possible. Progress in the analysis of variability and virulence patterns of the pathogen population is also necessary to test the
available germplasm with representative
isolates. Durability of the resistance can be
enhanced by appropriate cultural practices
and diversifying sources of resistance.
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Zhang, X., Halye, S.D. and Jin, Y. (2001) Inheritance of Septoria tritici blotch resistance in winter wheat.
Crop Science 41, 323–326.
7
Barley and Wheat Resistance
Genes for Fusarium Head Blight
S.A. Stenglein and W.J. Rogers
Laboratorio de Biología Funcional y Biotecnología (BIOLAB)-CEBB, Facultad
de Agronomía, Universidad Nacional del Centro de la Provincia de Buenos
Aires (UNICEN), Buenos Aires, Argentina and Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET), Argentina
Abstract
The genetic control of resistance to Fusarium head blight (FHB) in barley and wheat is reviewed. This
disease, which can reach epidemic proportions under certain climatic conditions, is caused by various
Fusarium species and affects grain yield and quality detrimentally, resulting in important economic
losses in both crops. Furthermore, FHB infection poses a serious threat to human and animal health,
due to the presence of toxic trichothecenes, of which deoxynivalenol and its derivatives appear to be
the most important. Marker-based mapping studies have identified numerous quantitative trait loci
(QTLs) for FHB resistance, located on all the chromosomes of both species. Only a relatively small
number of these can be detected consistently over a wide range of different environments and genetic
backgrounds. None the less, where genetic effects have been characterized, they have been shown to
be mainly additive in nature, meaning that the accumulation of several QTL factors in a single line
ought to be effective in achieving raised levels of resistance. Indeed, marker-assisted selection has
been directly shown to be feasible for some QTL. A number of QTLs for FHB resistance are associated
with other agronomic characters, such as heading date (HD), flowering time and plant height. In some
cases, QTL alleles favourable for resistance are associated detrimentally with alleles for these characters, although there appear to be sufficiently large numbers of QTLs for resistance acting independently of these characters to imply that reasonable genetic gains for resistance ought to be achievable
in the future. While most studies in barley have addressed Type I resistance (initial infection) and in
wheat Type II (spread between spikelets), or a combination of both Type I and Type II, more recent
studies have addressed other types of resistance, such as Type III (effects on kernel size and characteristics), Type IV (yield tolerance) and Type V (decomposition or non-accumulation of mycotoxins such
as deoxynivalenol). Besides identifying additional QTLs, these latter studies offer insights into the
mechanisms of the different types of resistance observed, in some cases blurring the distinctions
between them. Other prospects for improvement in FHB resistance, additional to those offered by
marker-assisted selection, are also discussed.
Introduction
Fusarium head blight (FHB) or scab is a
destructive disease of wheat and barley in
78
environments with prolonged wet climatic
conditions from flowering through the softdough stage of kernel development (Parry
et al., 1995; McMullen et al., 1997). The
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Barley and Wheat Resistance Genes
disease is of worldwide importance. FHB
epidemics have been documented in 26 US
states and five Canadian provinces. Economic losses in wheat since 1990 were estimated at US$2.5bn (Windels, 2000). Wheat
yields in 1993 were reduced by about 50%
in north-eastern North Dakota and 40% in
north-western Minnesota, compared with
1992 (National Agricultural Statistics Service, 1993–1999). Barley losses have been
equally devastating, with estimated losses
from 1993 to 1999 totalling in excess of
US$400m (Windels, 2000). In China, FHB
has affected more than 7m ha wheat and has
caused yield losses of more than 1 Mt in
severe epidemics (Leonard and Bushnell,
2003). In Argentina, during the past 60 years,
several FHB epidemics of varying severity
have occurred in the central-north area,
where yield losses were estimated to average between 20 and 50%.
FHB is a preharvest disease, but Fusarium species can grow in postharvest phase
if wet grain is not dried efficiently and
quickly. More than 17 Fusarium species
Fig. 7.1.
79
have been isolated from naturally infected
wheat or barley spikes and have been associated with FHB (Parry et al., 1995; Leonard
and Bushnell, 2003). Fusarium graminearum
(teleomorph Gibberella zeae) is the most frequently encountered pathogen and the most
virulent species, although F. avenaceum
(teleomorph G. avenacea), F. culmorum and
F. poae are reported to be prevalent in some
European and North and South American
countries (Leonard and Bushnell, 2003;
Barreto et al., 2004; Bourdages et al., 2006).
The distribution and predominance of a
Fusarium species in a region is thought to
be determined by climatic factors, competition among various Fusarium spp. sharing
the same ecological niches, fertilizer use,
cropping sequence and practices and vegetation type (Snyder and Nash, 1968; Nelson
et al., 1981; Doohan et al., 2003).
FHB reduces kernel set and kernel
weight. Invasion of the kernel by Fusarium
destroys the starch granules and cell walls
and affects endosperm storage proteins,
resulting in a poor quality product (Fig. 7.1).
Shrivelled lightweight seeds of wheat affected by FHB (left) and healthy wheat seeds (right).
80
S.A. Stenglein and W.J. Rogers
Germination rate and seedling vigour are
reduced when the seeds are infected.
In addition to causing significant yield
losses, FHB is of greater significance under
certain conditions because of the associated
mycotoxin accumulation which can occur
in infected grain. Fusarium graminearum,
F. avenaceum, F. culmorum and F. poae can
produce a range of mycotoxins and contaminated grain is unsuitable for animal and
human consumption because of the adverse
effects of such toxins on health (Placinta
et al., 1999; Gutleb et al., 2002). Within
Fusarium mycotoxins, some of the most
important from the point of view of animal
health and productivity, are the trichothecenes, zearalenone and the fumonisins
(D’Mello et al., 1999). Type A and B trichothecenes represent the most important
members of these mycotoxins. Type A trichothecenes include T-2 toxin, HT-2 toxin,
neosolaniol (NEO) and diacetoxyscirpenol
(DAS), while type B trichothecenes include
deoxynivalenol (DON, also known as vomitoxin) and its 3-acetyl and 15-acetyl derivates (3-DON and 15-DON, respectively),
nivalenol (NIV) and fusarenon-X (FUS-X).
A common feature of many Fusarium species is their ability to synthesize zearalenone (ZEN or F-2 toxin) and its co-occurrence
with certain trichothecenes raises important
issues regarding additivity and/or synergism in the aetiology of mycotoxicosis in
animals (Placinta et al., 1999). Fumonisins
are an increasingly important group of toxins as they have been postulated as the
causative agent for several endemic diseases, both in humans and animals (Sydenham et al., 1990; Chu and Li, 1994).
Host resistance has long been considered the most practical and effective means
of disease handling, but breeding for FHB
resistance has been hindered by a lack of
effective resistance genes and by the complexity of the resistance in identified sources
(Mesterhazy, 1997). No source of complete
resistance is known and current sources
provide only partial resistance.
Resistance types are generally classified
as either morphological or physiological.
Head anatomy or positioning that contributes to higher humidity around the spikelets
is often associated with more diseases. Generally, awned genotypes with short peduncule and a compact spike have faster disease
spread than genotypes that are awnless,
have a long peduncule and a lax spike (Rudd
et al., 2001). In addition, short saturated
genotypes with a long grain-filling duration
generally get more disease than tall genotypes that have rapid grain fill (Mesterhazy,
1995). These morphological characteristics
contribute to resistance, but are often considered nuisance factors in screening nurseries,
and it is generally agreed that they are of
minor significance compared with physiological resistance (Rudd et al., 2001). However, morphological traits have also been
associated with FHB resistance in barley.
Two-rowed barley is more resistant to FHB
than six-rowed barley and, in crosses between
six-rowed and two-rowed genotypes, tworowed progenies are most resistant, followed
by genotypes heterozygous for spike type.
Six-rowed types are most susceptible (Takeda
and Heta, 1989; Xihang et al., 1991).
Mesterhazy (1995) described five types
of physiological resistance, expanding the
two types described by Schroeder and Christensen (1963). These include Type I resistance to initial infection. It may be passive,
involving morphological characteristics of
wheat head. Alternatively, Type I resistance
may be active and include defence reactions
such as the activation of enzymes degrading
the fungal cell wall or pathogenesis-related
(PR) proteins (Nicholson et al., 2005). This
type of resistance is estimated by spraying a
spore suspension over flowering spikes and
counting diseased spikelets. Type II refers
to the resistance of movement of the pathogen from one infected spikelet to another via
the rachis. The mechanisms involved in
Type II resistance are thought to be active,
but again may be due to morphological characteristics. This type of resistance is estimated
by delivering conidia into a single floret of a
spike and counting the blighted spikelets after
a period of time. The other types of resistance
include: kernel size and number retention
(Type III), yield tolerance (Type IV) and
decomposition or non-accumulation of mycotoxins (Type V). Type III resistance is
measured by threshing infected spikes and
Barley and Wheat Resistance Genes
observing the damage to the kernels. Kernel
number reduction, kernel weight, test weight,
or visual estimates of Fusarium-damaged
kernels (tombstones) are common measurements used to assess this resistance. Type
IV resistance, or yield tolerance, can be
assessed by measuring grain yield of naturally or artificially inoculated spikes or plots
and comparing the data with spikes or plots
that do not show disease symptoms (Rudd
et al., 2001). Finally, Type V resistance is
identified by measuring DON concentration
at a given level of FHB (Rudd et al., 2001).
This resistance is important from a grain
utilization perspective, for example for
malting barley, because even trace levels of
DON may reduce beer quality significantly.
Considerable progress in the search for
host resistance has been made. Improvement of cultivar resistance has become a
major breeding objective worldwide. Recent
developments in genomic research and biotechnology hold promise for understanding
the genetic mechanisms of FHB resistance
and allow more effective utilization of FHB
resistance genes to develop new resistant
wheat and barley cultivars.
Genetics of FHB Resistance in Barley
Few sources of FHB resistance have been
found in barley and the level of their resistance is modest. Although FHB in barley
usually does not spread from spikelet to
spikelet within a spike (up and down the
spike), barley seems to be very susceptible
to initial infection. Severe disease usually
results from multiple initial infections in
the spike.
Of primary importance to barley breeders are data on FHB severity and DON concentration, since these are traits that affect
the marketing of grain in malting most
severely. The first sources of resistance used
were the breeding lines Gobernadora from
ICARDA/CIMMYT in Mexico and Zhedar 1
and Zhedar 2 from China. All three lines had
the two-rowed spike morphology. Other tworowed barley with low DON content were
CI 4196, Svanhals and Imperial. CI 4196
81
was identified as one of the most resistant
two-rowed barley accessions and also accumulated low concentrations of DON (Urrea
et al., 2005).
Six-rowed types are preferred for malting, but they are generally more susceptible
to FHB than two-rowed barley. Chevron, an
old cultivar from Switzerland, is a six-rowed
malting barley and a popular parent in barley breeding programmes. It has high resistance to kernel discoloration, which is a
disease complex caused by several different
fungal pathogens, including Fusarium.
In China and Japan, over 10,000 barley
accessions from different countries have
been screened for FHB resistance, but only
several dozen accessions have a low level of
FHB (Xihang et al., 1991; Zhou et al., 1991).
To date, no wild species of Hordeum have
shown greater resistance than that of tworowed barley. DON content in even the best
sources of resistance are still well above the
specification for the brewing industry
(< 0.5 mg/kg), but much lower than that of
current commercial malting barley cultivars
(Leonard and Bushnell, 2003).
Investigation of the genetics of resistance to FHB in barley has not been very
extensive and published reports on the identification of loci controlling FHB resistance
and DON accumulation are limited (Rudd
et al., 2001). Barley producers currently
attempt to manage the disease through crop
rotation and fungicide application. However, these measures alone are not sufficient
to reduce the risk of the disease. Resistant
barley cultivars are the most cost-effective
measures for controlling the disease, but
breeding for FHB resistance has been difficult for several reasons. One, genetic resistance is complex. There seem to be many
QTLs that have relatively small effects and
are subject to genotype × environment interactions. Two, FHB screening experiments
are labour-intensive and expensive. Three,
assessing FHB severity in both the field and
the greenhouse is difficult. Disease severity is
correlated strongly with HD and other agronomic and spike morphology traits. Since
infection can occur only after the spike
emerges from the boot, differences in HD
make it difficult to distinguish ‘true’ disease
82
S.A. Stenglein and W.J. Rogers
resistance from ‘apparent’ resistance that is
due to host escape from the pathogen. Both
of these problems necessitate the identification of molecular markers linked to QTLs for
FHB resistance that can be used in markerassisted breeding. In addition, since disease
expression is influenced strongly by the
environment, comparisons among barley
genotypes that differ in HD are themselves
confounded by the effect of the environment
on disease development. However, because
of the complex nature of genetic resistance to
FHB, QTL identification is not always very
robust. Therefore, validation of these QTLs is
important before implementing markerassisted selection in a breeding programme.
To gain a genetic understanding of FHB
resistance in barley, multiple sources of
resistance including Chevron (de la Pena
et al., 1999; Ma et al., 2000), Gobernadora
(Zhu et al., 1999), Fredrickson (Mesfin et al.,
2003; Smith et al., 2004), Zhedar 2 (Dahleen
et al., 2003) and CI 4196 (Horsley et al., 2006)
have been used in QTL mapping studies.
QTLs providing resistance to FHB and
DON accumulation in barley have been
identified on all seven chromosomes. QTLs
for FHB resistance were identified on chromosomes 1(7H), 2(2H), 3(3H), 4(4H), 5(1H)
and 7(5H) in the Chevron (resistant)/M69
(susceptible) population (de la Pena et al.,
1999). A major QTL on chromosome 2(2H)
explains 13.5% of the phenotypic variation
for FHB resistance. However, this QTL is also
associated with HD and the resistant allele is
linked to late heading. Ma et al. (2000) used a
population derived from the cross Chevron/
Stander and reported nine QTLs for FHB
resistance located on chromosomes 1(7H),
2(2H), 3(3H), 6(6H) and 7(5H). A QTL on
chromosome 2(2H) was detected consistently
in five environments and explained 11.8–
20.7% of the phenotypic variation for FHB
resistance. This QTL, in addition to the QTL
on chromosome 2(2H) discovered by de la
Pena et al. (1999), is also associated with days
to heading. Using a population derived from
the two-rowed parents, Gobernadora and
CMB 643, Zhu et al. (1999) found QTLs for
FHB resistance on all barley chromosomes
except chromosome 7(5H). The largest QTL
explained 33% of the phenotypic variation
and was found on chromosome 2(2H). The
QTL on chromosome 4(4H) explains 4–12%
of the phenotypic variation for FHB resistance. This QTL was also associated significantly with morphological traits including
plant height, seeds per inflorescence, inflorescence density and lateral floret size. In each of
the previous mapping studies, QTLs for accumulation of DON in harvested grain were also
detected. These QTLs were also distributed
throughout the genome and were, in some
cases, coincident with FHB QTL. Taken
together, these studies indicate resistance is
conditioned by many loci and that there is a
strong association between certain morphological traits and FHB resistance.
Two major traits associated with FHB
severity are spike type and HD. The Vrs1
and Int-c loci control lateral floret fertility
and hence determine whether a spike is tworowed (Vrs1; int-c/int-c) (Lundqvist and
Franckowiak, 1997) or six-rowed (vrs1/vrs1;
Int-c/Int-c) (Hockett and Nilan, 1985). In several studies, the two-rowed spike type has
been associated with FHB resistance (Chen
et al., 1991; Xihang et al., 1991; Steffenson
et al., 1996; de la Pena et al., 1999). In a genetic
study, Takeda (1990) demonstrated an association between the Vrs1 locus and FHB
resistance. In two-rowed barley (Vrs1) with
the Int-c/Int-c genotype, the laterals can be
inflated and lateral floret size has been associated with FHB severity (Zhu et al., 1999).
The FHB mapping studies published to date
have used populations derived from either
six-rowed × six-rowed or two-rowed × tworowed crosses (de la Pena et al., 1999; Zhu
et al., 1999; Ma et al., 2000). Therefore, the
Vrs1 locus was not segregating in these populations. HD may also strongly influence the
severity of FHB on barley and QTLs for HD
and FHB resistance are coincident (de la
Pena et al., 1999; Ma et al., 2000). Generally,
late heading plants tend to have lower severity, while early heading plants have higher
severity, indicating that the late heading
plants are exposed to the inoculum for a
shorter period of time (Leonard and Bushnell, 2003).
In all of these studies except the one
using Gobernadora, the bin 8 region of the
long arm of chromosome 2H designated by
Barley and Wheat Resistance Genes
Horsley et al. (2006) as Qrgz-2H-8 was associated consistently with FHB severity, HD
and DON concentration. The approximate
size for the overlapping QTL region ranged
from 22cM in the Fredrickson/Stander population (Mesfin et al., 2003) to 45cM in
Chevron/M69 (de la Pena et al., 1999) and
CI 4196/Foster (Horsley et al., 2006) populations. Depending on the population and
the environment, Qrgz-2H-8 explained 7–60%
of the variation in FHB resistance, 12–30%
of the variation in HD and 10–30% of the
variation in DON concentration. In all of the
studies, FHB severity and DON concentration were correlated negatively with HD. In
a validation study of this QTL, the Chevron
introgression at the Qrgz-2H-8 region reduced
FHB by 42% and increased HD by 3.8 days
(Canci et al., 2004).
The association between lower FHB
severity and late heading may be due to
shorter inoculum exposure (pleiotropy) or
tight linkage of separate genes for flowering
time and disease resistance (Leonard and
Bushnell, 2003). To determine if the association between late HD and FHB resistance is
due to linkage or pleiotropy, Nduulu et al.
(2007) constructed a fine map for the chromosome 2(2H) QTL region using recombinant
near isogenic lines (rNILs) derived from a
cross between a BC5 line carrying the Chevron alleles for markers at the Qrgz-2H-8 region
and the recurrent parent M69, and concluded
that the relationship between FHB and HD at
the Qrgz-2H-8 region was likely due to tight
linkage rather then pleiotropy.
Genetics of FHB Resistance in Wheat
Besides similar considerations as for barley regarding the detrimental effects of FHB
on grain yield and quality in general, and
the effects of mycotoxins on human and livestock health, the fact that the disease results
in the degradation of the endosperm storage
proteins means specifically that the quality
of bread, biscuit, pasta and other industrial
products can be seriously prejudiced. Worldwide, the species F. graminearum predominates, but F. avenaceum, F. culmorum and
83
F. poae are also the cause of the disease in
some environments. Epidemics may cause
major losses when climatic conditions are
favourable after flowering (Paillard et al.,
2004). As in barley, agricultural management
and fungicide treatments, while reducing the
damage (Gervais et al., 2003), are not wholly
effective (Stack, 1989; Bai and Shaner, 1994;
Parry et al., 1995). Unfortunately, complete
FHB resistance is unknown, although longterm control of the disease is probably most
likely to be achieved through genetic resistance research, involving QTL mapping and
other procedures (see below), and its consequent application in the breeding of resistant cultivars. This appears to be the case, in
spite of the complexity of the genetic control
involved, the presence of confounding environmental effects, the influence of genotype × environment interaction and the fact
that laborious inoculation and evaluation
procedures in mature host plants are required
in order to identify useful marker associations (Snidjers, 1990; van Ginkel et al., 1996;
del Blanco et al., 2003). A further complication is that associations between FHB resistance with HD, flowering time (FT) and
plant height (PH) have also been observed
(Mesterhazy, 1997; Hilton et al., 1999; Buerstmayr et al., 2000).
For breeding purposes, three broad origins of resistant germplasm have been recognized (Gilbert and Tekauz, 2000; Paillard
et al., 2004): (i) spring wheat from Asia (e.g.
cv. Ning 7840 [China], cv. Sumai 3 [China],
cv. Nobeokabozu [Japan]); (ii) spring wheat
from South America (e.g. cv. Frontana [Brazil]); and (iii) winter wheat from Europe
(e.g. Arina, Praag-8, Novokrumka). Further
examples of individual resistant cultivars
are given in the studies described below,
which are all concerned with bread wheat,
unless specified otherwise.
In contrast to barley, FHB generally
spreads between spikelets (although it is
currently unclear whether this is so for F.
poae) and most genetic research has therefore concentrated on Type II resistance (most
frequently evaluated after single-spikelet
inoculation with F. graminearum), although
combined evaluation of Type I and Type II
resistance through spray inoculation has
84
S.A. Stenglein and W.J. Rogers
also been widely carried out. However,
there are an increasing number of studies
that address other types of resistance, such
as the ability to detoxify DON (Type V) and
the ability to maintain grain yield in spite of
disease symptoms (Type IV).
The first QTL mapping studies were
carried out in the mid-1990s (Bai, 1995;
Moreno-Sevilla et al., 1997), involving the
use of RFLP and RAPD markers to map Type
II resistance. However, the marker associations identified individually accounted for
only a small proportion of the variation, perhaps due to the relatively low level of polymorphism observed for the markers employed
(Bai et al., 1999). Subsequently (Bai et al.,
1999), AFLP markers were applied to a mapping population involving the relatively
resistant cv. Ning 7840 (Type II resistant
cultivar), where the main specific character
measured was the area under disease progress curve (AUDPC) after F. graminearum
single-spikelet inoculation. One major QTL
was identified accounting for up to 60% of
the observed variation, which, although originally thought to be located on 7B, was identified subsequently as being equivalent to the
QTL identified on chromosome arm 3BS
(designated Qfhs.ndsu-3BS) (Waldron et al.,
1999) and present in one of the ancestral cultivars of cv. Ning 7840, namely cv. Sumai 3.
Two years later (Anderson et al., 2001), the
same group verified the presence of this QTL
(up to 41.6% of the variation accounted for) in
Sumai 3 and located two further QTLs from
Sumai 3 on 6AS (up to 11.6%) and 6BS (up to
9.2%). The susceptible parent, cv. Stoa, was
also shown to carry two QTLs for resistance,
on 2AL (up to 14.3%) and 4BS (up to 7.2%).
A further QTL from a third line, ND2603
(partially resistant), was located on 3AL (up
to 9.1%), in this case in a cross with the susceptible cv. Butte 86. These studies referred
to Type II resistance (0–100% FHB severity
scale after F. graminearum single-spikelet
inoculation).
During this period, in crosses between
six resistant Chinese bread wheat cultivars
with two susceptible cultivars (Bai et al.,
2001), where AUDPC was evaluated after F.
graminearum single-spikelet inoculation, it
was shown, from joint scaling tests aimed at
explaining the differences in means between
parental, F1, F2 and backcross generations
(Mather and Jinks, 1982), that most of the
observed genetic variation could be explained
by additive effects, where dominant and
epistatic effects accounted for only a small
proportion of the genetic effects present in
the crosses analysed. The authors pointed
out that this implied that it should be possible to accumulate different genes to improve
resistance to FHB. The mainly additive
nature of genetic effects was also observed in
the soft red winter wheat, Ernie (Liu et al.,
2005).
In a subsequent study involving Type II
resistance after inoculation with F. graminearum and F. culmorum (applied separately)
of a mapping population derived from the
bread wheat cross cv. CM-82036 (resistant,
a line derived from Sumai 3) × cv. Remus
(susceptible) and using RFLP, AFLP, SSR
and endosperm storage protein markers
(Buertsmayr et al., 2002), the large effect of
Qfhs.ndsu-3BS (up to 60% of variation
accounted for) was again confirmed and two
further QTLs were located to 5A and 1B.
The 3BS and 5A QTLs were flanked with
SSR markers and the 1B QTL associated
with the Glu-B1 locus encoding high molecular weight glutenin subunits. In a second
part of this study (Buertsmayr et al., 2003),
the authors extended the analysis to include
combined Type I and Type II resistance; they
found that, under spray inoculation, Qfhs.
ndsu-3BS had a much larger effect than the
5A QTL, which they named Qfhs.ifa-5A,
whereas after single-spikelet inoculation, the
two loci showed effects of similar magnitude. Qfhs.ndsu-3BS appeared to be associated mainly with resistance to fungal spread
(Type II), whereas Qfhs.ifa-5A appeared to
be associated principally with fungal penetration, and might contribute primarily
towards Type I resistance and, to a lesser
extent, towards Type II. In both these studies, no isolate × wheat genotype interaction
was observed, consistent with the previously observed non-specific or horizontal
nature of resistance (Mesterhazy, 1995; van
Eeuwijk et al., 1995), which was particularly interesting in this case since the two
isolates used belonged to different species
Barley and Wheat Resistance Genes
(F. graminearum and F. culmorum). The
authors concluded that FHB resistance
depended on a few (2–3) major QTLs, operating together with unknown numbers of minor
genes. They pointed out that marker-assisted
selection (MAS) for the major QTLs ought to
be a feasible method of accelerating the
development (through breeding that included
use of backcrosses) of resistant cultivars that
combined Type I and Type II resistance.
They felt that marker-mediated transfer of
the QTL to durum wheat also ought to be
feasible, given that no D genome chromosomes were involved in the QTL identified.
The effect of Qfhs.ndsu-3BS was also
observed in several other studies (Kolb
et al., 2001; Zhou et al., 2002; Bourdoncle
and Ohm, 2003; del Blanco et al., 2003;
Shen et al., 2003a; Xie et al., 2007). Effects
on 2A and 2B have also been observed in
analyses involving Sumai 3 (Zhou et al.,
2002). In one study (Yu et al., 2006), it was
suggested that the 3BS, 5AS and 6BS resistance QTLs of Sumai 3 were derived from
the Chinese landrace, Taiwan Xiaomai.
QTLs on chromosomes 2A, 3A, 3B and
5A, which had been observed previously in
Asian wheats, were also observed in RILs
derived from a cross between the European
winter wheat cultivars, Renan (resistant) and
Récital (susceptible), using spray inoculation
of F. culmorum (Gervais et al., 2003). In the
same study, new QTLs were identified on
2BS and 5AL. Although co-localization of
QTLs for resistance with awnedness (5A),
PH (5A) and FT (2B) was observed, the
authors considered that it should be possible to produce resistant lines independent
of these characters.
In RILs obtained from the Swiss winter
wheat cross cv. Arina (resistant) × cv. Forno
(susceptible) characterized with microsatellite and RFLP markers and subjected to spray
inoculation with F. graminearum (combined
Type I and II resistance), eight QTLs were
identified that together explained 47% of
the variation (Paillard et al., 2004). Three of
these were considered of major effect: 6DL
(22%), 5BL (14%, contributed by the susceptible parent) and 4AL (10%). The authors
considered that these were different from
QTLs previously reported. The other QTLs
85
detected were located on 2AL, 3AL, 3BL,
3DS and 5AL. The authors concluded that
FHB resistance was polygenic, rather than
the bimodal distribution observed in some
previous studies (Bai et al., 1999; Waldron
et al., 1999; Buertsmayr et al., 2002). The
2AL QTL was located at the same map position as one originating from cv. Stoa (Waldron et al., 1999; Anderson et al., 2001) and
the 5AL QTL in the same position as one
identified previously (Gervais et al., 2003).
In contrast, the 3DS QTL was located differently compared to one identified previously
on this arm (Shen et al., 2003a). The major
6D and 5B QTL overlapped completely with
a QTL for HD and the 6D QTL overlapped
partially with a QTL for PH. However, QTLs
for PH were identified that were not associated with FHB resistance. The data could
not distinguish pleiotropic effects from
linkage. A further study involving cv. Arina
(crossed to cv. NK93604) failed to detect the
same QTL (Semagn et al., 2007); instead,
QTLs on 1BL and 6BS from Arina and on
1AL and 7AL from NK93604 were detected.
A study of Arina crossed to the susceptible
UK cultivar, Riband, identified at least 10
QTLs, very few of which were coincident
with the other Arina studies; the most consistent was a major QTL on 4DS (Draeger
et al., 2007), detected in four of the five
environments evaluated.
In the winter wheat cross cv. Patterson × cv. Fundulea F201R (resistant cultivar
from Rumania), QTLs for Type II resistance
were found on 1B, 3A, 3D and 5A, with the
1B and 3A consistent over experiments
(Shen et al., 2003b).
It appears that, whereas Sumai 3 and its
derivatives have major QTLs on 3B and 5A,
the three winter wheat populations so far
characterized seem to depend more on the
accumulation of moderate and minor QTLs.
The 3BL QTL located in the Renan/Récital
population may be the same as that observed
in the Arina/Forno population.
In a cross of the resistant Brazilian cv.
Frontana with the susceptible cv. Remus
(Steiner et al., 2004) inoculated with F.
graminearum and F. culmorum, a major
QTL accounting for 16% of the variation in
FHB severity and incidence was located on
86
S.A. Stenglein and W.J. Rogers
3A and a QTL accounting for 9% of the variation in FHB severity was located on 5A.
Smaller effects for severity were located on
1B, 2A, 2B, 4B, 5A and 6B. The resistance of
Frontana was found to be due principally to
the inhibition of fungal penetration (Type
I), but with a minor effect on fungal spread
(Type II). PH, FT and spike morphology
influenced FHB reaction, but co-localization
of QTLs was observed only for minor QTL,
and sufficient QTLs for FHB resistance acting independently of these characters were
observed in order to allow selection of resistant lines with any height, flowering date
and spike morphology.
Seven QTLs for Type I and II resistance
were found on 1BS, 1DS, 3B, 3DL, 5BL, 7BS
and 7AL in a cross between cv. Cansas
(moderately resistant) and cv. Ritmo (susceptible). The 1DS QTL seemed primarily to
involve resistance to fungal penetration,
while the other QTLs were concerned mainly
with resistance to fungal spread (Klahr
et al., 2007). Significant correlations with
PH and HD were observed.
The Qfhs.ndsu-3BS region of Sumai 3
has been fine mapped and named Fhb1
(Cuthbert et al., 2006), as well as being validated by near-isogenic line studies (Cuthbert et al., 2007). A second region on 6BS
has also been fine mapped and named Fhb2
(Pumphrey et al., 2007).
Over recent years, attention has turned
towards other types of resistance. For example, in the partially resistant cultivars,
Wuhan-1 and Maringa, QTLs for the accumulation of DON (Type V) were located on
2DS and 5AS (as well as QTLs for FHB resistance on 2DL, 3BS and 4B) (Somers et al.,
2003). QTLs were located on 5A (12.4%),
2A (8.5%) and 3B (6.2%) for low DON content in the Chinese landrace, Wangshuibai
(as well as QTLs for Type II resistance on 3B
and 2A (Ma et al., 2006)). In the previously
cited study on Arina × NK93604 (Semagn
et al., 2007), the QTLs located on 1AL and
2AS were associated with DON content,
although only 1AL was associated with FHB
resistance. In the additional Arina study
cited, involving Arina × Riband (Draeger
et al., 2007), the major 4DS QTL identified
was found to affect AUDPC, DON content,
fungal DNA content (FDNA), relative spikelet weight (RSW) and per cent of Fusariumdamaged kernels (FDK); although this may
be due to linked genes, the authors considered it more likely to represent one resistance gene (which appeared to be linked to
the Rht-D1 locus, an association that may
prejudice attempts to improve resistance in
germplasm containing the Rht-D1b (Rht)
semi-dwarfing allele). In this study, further
QTLs were observed as follows, whose
detected presence varied over environments:
AUDPC: 1BL, 2B, 6BL, 7AL, 7BL, 7DL; DON
content: 6BL, 7DL; FDNA: 3DL, 6BL, 7BL;
RSW: 1BL, 2AS, 6BL, 7DL; FDK (Type III):
5AS, 7AL; yield loss (Type IV): 7AL.
In a study involving lines derived from
the cross CM-82036 × Remus (Lemmens
et al., 2005), the QTL on 3BS derived from
Sumai 3, closely associated with resistance
to spread of the disease (Type II), appears to
convert DON to DON-3-O-glucoside. The
authors hypothesized that the 3BS QTL
encoded a DON-glucosyl-transferase or regulated the expression of this.
In a cross involving cv. CJ 9306 (Jiang
et al., 2007), two QTLs were found for resistance to DON accumulation, QFhs.nau-2DL,
explaining up to 20% of the observed variation, and QFhs.nau-1AS, explaining 4–6%.
The QTLs, QFhs.ndsu-3BS (up to 23% of
the variation) and QFhs.nau-5AS (4–6%)
were also validated. QTL × environment
interaction was found for QFhs.nau-2DL
only. The authors suggested that marker-assisted selection would be effective and made
suggestions for the particular markers to be
used, either singly or in combination. They
also validated QFhs.ndsu-3BS for resistance
to grain yield loss (Type IV). No QTL independent of Type II resistance was found.
In many of the above studies, markers
closely linked to the FHB resistance QTL
were identified, enabling MAS to be contemplated. For example, SSR markers for
the 3A and 5A QTL in Frontana have been
identified, allowing these to be combined
through MAS with the QTL in Sumai 3 and
its derivatives. The feasibility of MAS has
been directly demonstrated (Wilde et al.,
2007), involving the 3B and 5A resistance of
Sumai 3 and the 3A resistance of Frontana;
Barley and Wheat Resistance Genes
MAS for the two Sumai 3 QTLs gave significant reductions in FHB severity and DON
content, although MAS for the Frontana
QTL had no effect. Additional phenotypic
selection acting on other unmarked QTLs
should give additional gain. Some markers
have been used extensively in breeding programmes (Guo et al., 2006).
Some of the above reports are particularly illuminating, since they appear to be
showing that the various types of resistance
are not necessarily truly distinct categories.
For example, the Sumai 3 3BS resistance
generally has been regarded as being of
Type II. However, this locus may in fact be
involved in detoxifying DON (Type V resistance). That is, it may be that at least a part
of the mechanistic basis of the Type II resistance associated with this locus is its Type
V nature.
The map-based cloning of QTL ought to
contribute to understanding resistance mechanisms further (Liu and Anderson, 2003;
Shen et al., 2006). An expressed sequences
tag (EST) rich in leucine and with low similarity to a protein kinase domain of the Rpg1
gene in barley was identified on 3BS and
might represent a portion of a gene for FHB
resistance (Shen et al., 2006). This EST
could be used in MAS and for map-based
cloning. Resistance gene analogues (RGA)
associated with 1AL have been identified
(Guo et al., 2006); all RGA markers studied
contained a heat shock factor that initiated
the production of heat shock proteins. Other
promising areas for improvements in FHB
are: (i) the introduction of genes from related
species (QTLs for FHB resistance have been
identified on 3A in Triticum dicoccoides
(Otto et al., 2002) and on 4A in T. macha
(Steed et al., 2005)); and (ii) the genetic
engineering of FHB resistance by, for example, the expression in wheat of Arabidopsis
NPR1 (Makandar et al., 2006).
The above studies (and others not
included here due to space confines, some
of which are cited in the ‘Catalogue of gene
symbols for wheat’ [Mclntosh et al., 2003]
and subsequent annual supplements published in the Annual Wheat Newsletter)
demonstrate that QTLs for FHB resistance
have been identified on all the chromosomes
87
of wheat, whose detected presence and magnitude of effects depend greatly on environmental factors and the particular genetic
background in which they are evaluated. In
this sense, the genetic control of resistance
appears to be complex, even though genetic
effects appear to be mainly additive in
nature. The situation may be set to become
more complicated still: although, as mentioned previously, FHB resistance is thought
to be non-specific or horizontal, recent studies indicate that interactions may be more
complex (Xihang et al., 1991).
Conclusions
Although handling of FHB requires the
application of several different disease management strategies, substantial progress has
been made in understanding the genetic
basis of resistance to FHB in wheat and barley. Quantitative resistance usually is caused
by the simultaneous segregation of several to
many genes and diverse non-genetic factors.
Of the several types of resistance that have
been hypothesized or reported, Type II
resistance is the most stable and well studied. The Chinese wheat cultivar, Sumai 3,
and its derivates are one of the best sources
of resistance to FHB and may provide the
maximum degree of Type II resistance. The
major QTL on chromosome 3BS is found in
most of the resistant cultivars from China.
However, QTLs located on all the other
chromosomes have also been reported but,
for many of them, their expression is not
stable over different environments or in all
genetic backgrounds.
Only a few barley cultivars have a relatively higher level of FHB resistance. Most
of these resistant cultivars are two-rowed
barley. Within six-rowed barley, which is
preferred for malting, the cultivar, Chevron,
has the best degree of resistance, but its
DON level is still too high and far from
meeting the safety requirements of the brewing industry. In contrast to wheat, Type I
resistance is the major resistance type in
barley. Molecular mapping indicates that
many QTLs, spread over many chromosomes
88
S.A. Stenglein and W.J. Rogers
and with minor effects, control this resistance. Correlation between FHB severity
and other spike-related traits has presented
a major barrier to breeding for FHB resistance in barley. Using MAS for the Chevron allele at the Qrgz-2H-8 locus should
help breeders surpass this barrier (Nduulu
et al., 2007).
Marked-assisted selection may provide
such a technique for dissecting and stacking
different resistant QTLs for FHB resistance
and the application of high-throughput markers for FHB-resistant QTLs may improve
selection efficiency significantly. Moreover,
recent developments in genomics and biotechnology hold promise for understanding
the genetic mechanism of FHB resistance
and for more effective development of resistant wheat and barley cultivars. Functional
genomics tools such as microarray analysis
and ESTs open a new way for genome-wide
gene expression profiling.
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8
Sustainable Management of Rice Blast
(Magnaporthe grisea (Hebert) Barr):
50 Years of Research Progress in
Molecular Biology
S. Nandy,1 N. Mandal,2 P.K. Bhowmik,1 M.A. Khan3 and S.K. Basu4
1Bioproducts
and Bioprocesses, Lethbridge Research Center, Agriculture and
Agri-Food Canada, Lethbridge, Canada; 2Bidhan Chandra Krishi Vishavidalay,
Nadia, India; 3Department of Weed Science, NWFP Agricultural University,
Peshawar, Pakistan; 4Department of Biological Sciences,
University of Lethbridge, Lethbridge, Canada
Abstract
Rice blast fungus (Magnaporthe grisea (Hebert) Barr) as a species has a very broad host range, infecting
more than 40 Graminaceous hosts and some other non-grass hosts. The seedling stage, the rapid tillering stage after transplanting and the flower emergence stage have been identified as the most susceptible to rice blast. In developing countries, poor farmers cannot afford to control blast disease by the
application of expensive fungicides. Therefore, sustainable rice blast disease management is more
important for environmental concern, as well as for better financial returns to farmers in Third World
countries. During the past few decades, a substantial amount of research has been conducted all over
the globe to cope with blast fungus. In this chapter, we emphasize specifically the molecular biological
aspect of the study on rice blast fungus over the past 50 years.
Abbreviations used: BRV: blast-resistant varieties; HR: hypersensitive response; RBD: rice blast
disease; RBF: rice blast fungus; RGAs: resistance gene analogues; ROI: reactive oxygen intermediates;
PCR: polymerase chain reaction; RAPD: random amplification of polymorphic DNA; RFLP: restriction
fragment length polymorphism.
Introduction
Many rice researchers consider blast to be
the most important disease of rice worldwide
(Valent and Chumley, 1994). This is because
the disease is widely distributed (85 countries) and can be very destructive when
environmental conditions are favourable.
Rice blast causes between 10–30% yield
losses worldwide in rice, posing a constant
92
threat to the supply of this staple food for
nearly one-half of the world’s population
(Zhu et al., 2000; Talbot, 2003). The rice
blast fungus (RBF), scientifically known as
M. grisea (Hebert) Barr (anamorph: Pyricularia grisea Sacc.), is a filamentous Ascomycetous fungus that parasitizes over 40
grasses, including economically important
crops like wheat, rice, barley and millet
(Ou, 1985), but the pathogen is best known
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Sustainable Management of Rice Blast
as the casual agent of the rice blast disease
(RBD). RBD is one of the most serious diseases in all rice-growing regions of the world.
Under heavy dew, all aerial parts of the plant
can be affected; leaf surfaces become speckled with oval to globular lesions and severely
infected plants are liable to lodging if stems
are infected. The infected panicle results in
severe yield loss (Ou, 1985). The fungus has
the capacity to overcome resistance in a
short period of time, soon after the release
of a resistant cultivar, and thus has made
breeding for resistance a constant and difficult challenge to address for rice breeders
and pathologists (Shao et al., 2008). Analysis of the existing genetic variation in plant
pathogen populations is an important prerequisite for understanding the mechanism
of co-evolution in the plant pathological system (McDonald et al., 1989). Several populations of rice blast pathogen all over the globe
have been studied for their characteristic
phenotypic and genotypic variations (Levy
et al., 1991, 1993; Shull and Hamer, 1994;
Chen et al., 1995; Kumar et al., 1999). Blast
disease was first reported in China (1637)
and then in Japan (1704), Italy (1828) and in
the USA (1996) (Asuyama, 1965; Ou, 1985;
CRRI Annual Report, 2001–2002). In this
chapter, we discuss the 50 years of research
on M. grisea and the available sustainable
disease resistance management in rice.
Epidemiology of Blast Disease
Seedling stage, rapid tillering stage after
transplanting and flower emergence stage
were identified as the most susceptible to
rice blast. The fact that the age of the leaves
influences the susceptibility to blast was also
brought out. The older the leaves on the plant,
the more they are resistant to blast (Ou, 1985;
CRRI Annual Report, 2001–2002). Excessive
exposure to nitrogen and cold night temperatures predisposed susceptible varieties, but
did not show any effect on highly resistant
varieties. The critical range of temperature
for penetration and establishment of infection was around 25–26°C, whereas germination of spores and appressoria formation
93
occurred within 6–10 h at 20–30°C in the
presence of water on the surface of the leaf
(Asuyama, 1965; Ou, 1985). The formation
of dew or a little rainfall or the occurrence
of fog provided the necessary water required
for the germination of spores. Analysis of
the intensity of infection recorded in different long-term experiments of several years
revealed that blast infection had occurred
under natural conditions when the minimum temperature during the night was
26°C and below, with the concomitant
occurrence of relative humidity of 90% and
higher (CRRI Annual Report, 2001–2002).
Grouping of Blast Fungal Isolates
M. grisea as a species has a very broad host
range, infecting more than 40 Graminaceous
hosts and some other non-grass hosts (Asuyama, 1965; Ou, 1985). Ou (1980) studied
variability in the pathogen and the host resistance of M. grisea. Monoconidial cultures
showed continued segregation for virulence
pattern and generated diverse lesion types
on individual leaves. Conidial and mycelial
cells of M. grisea were reported to contain
nuclei with a different number of chromosomes. These observations offered the best
genetic explanation for the variation. Latterell and Rosi (1986) studied the longevity and
pathogenic stability of M. grisea for 30 years.
They suggested that the species comprised a
wide range of pathotypes (races), each characterized by its capacity to attack certain
cultivars of rice, and that these races were
basically stable and mutations (or parasexual
recombination) were the exception rather
than the rule, resulting in broader host range
or increased sporulating capacity. The detection of parasexual DNA exchanges in wildtype strains and the existence of merodiploids
in nature suggest that parasexual recombination occurs in field populations of M. grisea
(Zeigler et al., 1997).
Three DNA probes were developed by
Hamer et al. (1989), which reliably and specifically identified the genetic backgrounds
of the full spectrum of the rice blast fungal
pathotypes. One of these probes consists of
94
S. Nandy et al.
cloned fragments of repeated DNA obtained
from the RBF genome and which are called
MGR586 (M. grisea repeat elements, previously referred to as PCB586). The probe
hybridizes with approximately 50 EcoRI fragments, ranging in size from 1.5–20.0 kb in the
genome of all M. grisea isolates pathogenic to
rice. Worldwide conservation of MGR586
sequences in RBF suggests that they descend
from a common ancestral source, genetically
isolated from other host-limited forms of M.
grisea. The use of MGR shows that sequences
are dispersed randomly on all chromosomes
of the pathogens and segregate as genetic loci
(Zeigler et al., 1997; Suzuki et al., 2007).
Borromeo (1990) studied the Philippine
isolates of RBF with MGR586 and MGR613.
Valent and Chumley (1994) discussed the
recent application of tools for molecular
genetic analysis of M. grisea and past and current research in the problem areas. Iwano
(1990) and Chen (1993) reported that the
racial composition in a field in Yongnan
province, China, and the Philippines showed
wide yearly fluctuations. Iwano (1990)
claimed that isolates from the same lesion
changed their reaction on a set of several cultivars annually. Silue et al. (1992) studied the
patterns of inheritance of avirulence in M. grisea in seven different rice cultivars. Avirulence to four cultivars has been reported as
being controlled by one gene, whereas for
the other three cultivars, it was controlled
by two genes.
In another study using DNA polymorphism, common ancestral patterns were
found among Magnaporthe infecting rice
isolates and their associated weed hosts (Borromeo et al., 1993) However, the pathogenic
populations infecting the weed hosts do not
supply pathogenic inoculums for the rice.
Weeds can act as alternative hosts for the
disease in greenhouse tests; but their role in
the field is not yet quite clear (Kato, 2001).
Rice, as a widely and intensively cultivated
crop, could be a potential target for parasitic
‘host shifts’ and a potential agent for ‘shifts’
to accompanying weeds (Couch et al., 2005).
The authors also reported the single origin
of rice-infecting M. oryzae after a ‘host shift’
from a Setaria-millet and that it was probably closely followed by additional ‘shifts’ to
weeds of rice, cutgrass and torpedo grass.
Levy et al. (1993) studied the genetic diversity of RBF in a disease nursery in Colombia. DNA fingerprints using MGR586, 115
haplotypes from 151 fungal isolates were
identified and partitioned into six discretely
distinct genetic lineages. Xia et al. (1993) conducted a DNA fingerprinting study to examine microgeographic variations in the M.
grisea population in two different rice fields
in Arakans in South-east Asia. The DNA fingerprints of 113 isolates were grouped based
on restriction fragment length polymorphism
(RFLP) similarity. Seven distinct fingerprint
groups were identified and four fingerprint
groups were common in both fields.
A study examining the relationship
between phylogeny and pathotypes for isolates of the RBF in the Philippines revealed
that the distribution of virulence was nonrandom with respect to lineage for the cultivars under study (Zeigler et al., 1995). Sivaraj
(1995) reported six different lineages (L, A, B,
E, F and H) from Karnataka in southern India,
using the MGR DNA fingerprinting approach.
The repetitive DNA element, MGR586, has
been widely used for fingerprinting and phylogenetic analyses of M. grisea. George et al.
(1998) developed a polymerase chain reaction (PCR)-based marker to DNA fingerprint
the Magnaporthe species coming from different biogeographic zones. Roumen et al.
(1997) studied the genetic variability among
41 isolates of the blast pathogen from five
rice-growing countries from the European
Union, including Spain, France, Hungary,
Italy and Portugal. DNA fingerprinting grouped the isolates into five discrete lineages,
which typically showed less than 65% band
similarity. Srinivasachary et al. (1998) classified 27 single spore isolates of M. grisea
from Karnataka in southern India over three
different locations using random amplified
polymorphic DNA (RAPD) primers. They
found three clear groups at 70% similarity
level. But Srinivasachary et al. (2002a,b) used
27 isolates from Ponnampet, Mandya and
Bangalore for genetic analysis using 30
RAPD primers. Three distinct lineages
were reported by the authors. Chadha and
Gopalakrishna (2005) also used 20 isolates
from seven different locations in India using
Sustainable Management of Rice Blast
123 RAPD primers for cluster analysis. Scientists have sequenced the M. grisea genome
and it is now available online at http://
www-genome.wi.mit.edu/annotation/fungi/
magnaporthe/. It is, however, important to
note that for the first time in the USA, the
genomic structure of a significant plant
pathogen has been made publicly available.
Physiology of Disease Resistance
Plants develop defence mechanisms to recognize pathogens and protect them from
attack. These defence reactions are triggered
by the recognition of pathogens by plant
disease resistance (R) genes. After the recognition of pathogens, a signalling pathway is activated, resulting in resistance to pathogens
(Hammond-Kosack and Jones, 1997). During the early steps in R gene-mediated disease resistance, reactive oxygen intermediates
(ROI) such as O2– and H2O2 are generated
rapidly after infection; and, subsequently,
hypersensitive response (HR) leading to cell
death has been observed. An understanding
of how pathogens induce disease, how the
plants become diseased and how they defend
themselves against the pathogens would
help us to understand the functions of the
genes governing resistance, which remains
unknown, and eventually to develop novel
methods for controlling RBD. The nature of
resistance to blast disease operating at both
the pre- and post-penetrative stages of the
disease was investigated using several models involving cultivars differing in their
reaction to the disease, nitrogen fertilization
and temperature-induced tissue susceptibility and resistance induced by certain
chemicals (CRRI Annual Report, 2001–2002).
Four different mechanisms govern blast resistance in rice: (i) the epicuticular wax present
on the surface of the leaves influences the
infection by suppressing the appressorium
formation by the pathogen, thus offering a
partial resistance resulting in a reduced
number of lesions being formed; (ii) free phenolic compounds and their oxidases toxify
the tissue in the infected region: the speed
and magnitude at which the toxification
95
takes place in response to infection determines the tissue resistance to the pathogen;
(iii) the presence of two toxic cinnamate
derivatives (ferulate and coumarate) in the
cell walls forming toxic oxidized products/
polymers like lignin and melanin-like compounds on oxidation forming a mechanical
barrier for the fungus and thereby arresting
the spread of the pathogen to adjacent cells,
thus restricting disease lesions; and (iv) the
synthesis and accumulation of antimicrobial
compound(s) (diterpenoid in nature) known
as ‘phytoalexins’ in response to infection
toxic to the growth of the pathogen. However,
none of these mechanisms seemed to be universal in nature and the defence mechanism
was dependent on the varieties tested (CRRI
Annual Report, 2001–2002).
Finding the Right Gene
The generation of cultivars that possess
non-specific resistance to M. grisea would
provide an economically effective and environmentally sound approach to rice blast
control. One promising approach to the
achievement of non-specific resistance to M.
grisea is to incorporate genes that elicit general defence responses in rice (Dang and
Jones, 2001; Stuiver and Custers, 2001).
Much effort has been devoted to understanding the genetic and molecular basis of resistance in RBF and several genes have been
cloned (Parson et al., 1987; Leung et al.,
1990; Khang et al., 2008; Shao et al., 2008).
Although earlier studies focused on
pathotypic variability (Ou, 1985), later studies focused extensively on molecular markers
to characterize population diversity (Nandy
et al., 2004). Extensive use of the MGR586
heterodispersed element (Roumen et al.,
1997; Kumar et al., 1999; Correll et al., 2000;
Viji et al., 2000; Srinivasachary et al., 2002a,b;
Chadha and Gopalakrishna, 2005) to delineate DNA fingerprint lineages has helped to
identify and classify the genetic structure of
this important pathogen. PCR-based molecular markers are useful tools for detecting
genetic variation within populations of
important plant pathogens (Vakalounakis and
96
S. Nandy et al.
Fragkiadakis, 1999; Kolmer and Liu, 2000;
Srinivasachary et al., 2002a,b; Chadha and
Gopalakrishna, 2005). RAPD (Welsh and
McClelland, 1990; Williams et al., 1990)
and markers have been widely used for estimating genetic diversity in wild populations
(Annamalai et al., 1995), mainly because the
technique does not need previous molecular
genetic information and increases marker
density for evaluating genetic kinship. The
RAPD technique has also been used to study
genetic diversity among RBF from different
geographical locations in the world (Lima,
1999; Suzuki et al., 2007).
The dynamic virulence of the rice blast
pathogen could be the main cause for the
breakdown of resistance in several rice varieties. The diversity and variability of the
pathogen population may originate from the
clonal mode of reproduction, coupled with
mutation, migration, selection or random
drift, heteroploidy and parasexuality of the
fungus (Gesnovesi and Magill, 1976; Dayakar et al., 2000; Noguchi et al., 2007). A
repeat sequence termed MGR586 was identified in the genome of rice-infecting strains
of M. grisea (Shull and Hamer, 1994). This
sequence has been widely used for DNA fingerprinting of M. grisea to investigate the
epidemiology of the RBD (Roumen et al.,
1997; Kumar et al., 1999; Correll et al., 2000;
Viji et al., 2000; Chadha and Gopalakrishna,
2005). Molecular analysis of isolates of M.
grisea from different regions within a state
(West Bengal, India) revealed the occurrence of a high level of polymorphism, indicating a wide and diverse genetic base
(Mandal et al., 2004). Overall, a high genetic
diversity was also obtained in Indian RBF
(Roumen et al., 1997; Kumar et al., 1999;
Correll et al., 2000; Mandal et al., 2004,
Chadha and Gopalakrishna, 2005).
Genetic mechanisms, namely simple
mutations, meiotic recombination and parasexual recombination, could explain such
genetic diversity (Yamasaki and Niizeki, 1965;
Zeigler, 1998; Zeigler et al., 2000, Khang,
2001). Some indirect evidence suggests that
M. grisea has the potential for sexual reproduction in specific geographic zones and
localities (Viji et al., 2000, Adreit et al.,
2007). There have been few investigations
on the perfect state of M. grisea in India
(Dayakar et al., 2000; Mandal et al., 2004).
The sexual cycle does not seem to be a
source of variation for the rice blast pathogen in India (Kumar et al., 1999). Similar
results have also been reported from other
corners of the globe (Valent et al., 1986).
The wide range of diversity among collected
isolates of M. grisea from different locations
in West Bengal can be explained mainly by
evolution resulting from natural and stressinduced transposition (Ikeda et al., 2001).
Other mechanisms like horizontal gene
transfer between RBF and its host (Kim et al.,
2001) may also be of importance because
varieties deployed within a region are based
on crop seasons, along with several other
biotic and geographic factors (Babujee and
Gnanamanickam, 2000).
Using Genetic Diversity
of Disease Resistance
Genetic studies of qualitative resistance
were started when Goto (1970) established
the differential system for races of P. grisea
or M. grisea in Japan. Thirteen major genes
for qualitative resistance have been reported
by several researchers (Kiyosawa et al.,
1981). Several rice cultivars with durable
blast resistance have been identified and
‘Moroberekan’ have been cultivated in the
world for many years without high losses
from blast (Notteghem, 1985). These plants
have been used as resistance donors in breeding programmes. Major resistance genes have
been used successfully for developing blast
resistance cultivars (Khush, 2004) and several dominant resistance genes have been
identified which confer complete blast resistance (Kiyosawa et al., 1981). Atkins and
Johnson (1965) identified two independent
genes designated Pi-1 and Pi-6. Hsieh et al.
(1967) in China found four dominant genes
for pathogen resistance in japonica cultivars,
named as Pi-4, Pi-13, Pi-22 and Pi-25 using
RFLP techniques. Yu et al. (1991) mapped
three major resistance genes, namely Pi-1,
Pi-2 and Pi-4 in the Philippines. Several genes
from tropical cultivars like ‘Tetep’, ‘Pai-kan
Sustainable Management of Rice Blast
tao’, ‘5173’, ‘LAC23’, Moroberekan and
‘Apura’ were identified and mapped using
RFLPs (Yu et al., 1991; Miyamoto et al.,
1996; Rybka et al., 1997) (Table 8.1). Recent
reports identified at least four clusters, with
five to eight loci each, located on chromosomes 4, 6, 11 and 12 (Roumen et al., 1997;
Rybka et al., 1997, Tabien et al., 2000; Gao
et al., 2002).
Many pathogenic races have been identified in M. grisea and pathogenic variability has been cited as the principal cause for
the breakdown of resistance in rice varieties
(Baker et al., 1997). Therefore, an artificial
inoculation study can be practised in place
97
of natural screening, which is quite cumbersome, time-consuming and season specific.
There has been considerable achievement
in the development of blast-resistant varieties
(BRV), particularly using vertical-resistant
genes (Nandy et al., 2004). Nevertheless,
durable resistance alone can protect irrigated rice crops in the tropics adequately.
Exploitation of durable resistance has been
proposed for less blast-conducive environments (Buddenhagen, 1983; Notteghem,
1985; Parlevliet, 1988; Bonman et al., 1992).
Artificial inoculation in Karnataka, southern India, was also carried out by Srinivasachary et al. (2002a) to study involving the
Table 8.1. List of blast disease-resistance genes with chromosome numbers, donor varieties and linked
markers of rice.
Gene
symbol
Chromosome
number
Donor variety
Pi-1(t)
11
Pi-2(t)
6
Pi-4(t)
12
Pi-5(t)
4
Linked marker
Reference(s)
LAC23, C101LAC
Npb181, RZ536
BL245, C101A51, 5173
RG64
Tetep, Pai-kan-tao,
BL245, C101PKT
RG869, RZ397
RIL 45, RIL 249,
Moroberekan
–
RG498
RG103
RG16
Atkins and Johnson (1965);
Yu et al. (1991); Leung et al.
(1998)
Yu et al. (1991);
Sridhar et al. (1999)
Yu et al. (1991); Hittalmani
et al. (1995); Tabien et al.
(2000)
Wang et al. (1994);
Sridhar et al. (1999)
Causse et al. (1994);
Atkins and Johnson (1965)
Wang et al. (1994)
Leung et al. (1998);
Khush et al. (1999)
Naqvi et al. (1995),
Tabien et al. (2000)
Zhu et al. (1992); Roca et al.
(1996); Khush et al. (1999)
Khush et al. (1999)
Miyamoto et al. (1996);
Khush et al. (1999)
Fukuoka and Okuno (1997);
Leung et al. (1998);
Sridhar et al. (1999)
Chao et al. (1999); Bryan
et al. (2000)
Shigemura and Kitamura
(1954); Rybka et al. (1997);
Leung et al. (1998); Bryan
et al. (2000)
Pi-6
12
RG869
Pi-7(t)
Pi-9
11
6
Pi-10
5
Moroberekan, RIL 29
O. minuta derivative
WHD-IS-75-1-127
Moroberekan
Pi-11
8
Oryzica Llanos 5
RRF6, RRH18,
OPF6(2700)
BP127, RZ617
Pi-12
Pi-b
12
2
Moroberekan, RIL 10
F-145-2
RG869
RZ123
Pi-z5
6
C101A51
RG64, RG612
Pi-k
11
F-129-1
–
Pi-ta and
Pi-ta2
12
Taducan, C101PKT,
IR64, F-124-1,
F128-1
RZ397, RG241
98
S. Nandy et al.
reaction of representative single-spore culture PPT-4 to rice varieties Moroberekan,
isolines of Co39, namely Pi-1, Pi-2, Pi-4,
Pi-2 + Pi-1, Pi-1 + Pi-4, along with IRAT177,
Apura and Doddi showed resistant reaction.
Of these, Pi-1, Pi-2, Pi-4, Pi-2 + Pi-1 and
Pi-1 + Pi-4 are known to contain major
genes conferring resistance to blast disease.
Yamada et al. (1976) and Kiyosawa et al.
(1981) selected 12 differential varieties for
resistance genes Pi-ks, Pi-a, Pi-k, Pi-km, Pi-z,
Pi-ta (Pi-4), Pi-ta2, Pi-zt, Pi-kp, Pi-b and Pi-t.
These differential varieties were used in
Japan especially, but were not readily available in other countries. Monogenic lines
including only a single gene in each genetic
background and targeting for 24 different
resistance genes – Pi-a, Pi-b, Pi-i, Pi-ks, Pi-k,
Pi-k-h, Pi-km, Pi-kp, Pi-sh, Pi-t, Pi-ta (Pi-4),
Pi-ta2, Pi-z, Pi-z5 (Pi-2), Pi-zt, Pi-1, Pi-3,
Pi-5(t), Pi-7(t), Pi-9, Pi-11(t), Pi-12(t), Pi-19
and Pi-20 – were developed by Tsunematsu
et al. (2000) as the first international standard differential variety set. The polymorphic RG-64 marker was used by Hittalmani
et al. (2001) to identify rice plants carrying
Pi-2(t) from an F2 population derived from
the cross between Co39 and C101A51. More
than 30 blast-resistant genes (Babujee and
Gnanamanickam, 2000) and QTLs have been
identified in rice by conventional genetic
studies based on linkage analyses and recombination frequencies (Kinoshita, 1991; Mackill et al., 1993). Some major genes for blast
resistance have been identified in recombinant inbred lines (RILs) (Wang et al., 1994).
Zeigler et al. (1995) proposed that organization of the blast fungus population into
well-defined lineages and their distribution
in specific geographic locations have led to
the employment of resistance genes targeted
against pathogen populations prevalent in
that region. This has been known as the ‘lineage exclusion’ hypothesis. Sivaraj et al.
(1996) proposed a model to support gene
pyramiding based on lineage exclusion. They
consider traditional plant breeding as a strategy of pathotype exclusion, which leads to
frequent resistance breakdown when appropriate pathotypes appear within 1 or 2 years
after such resistance is deployed in large
areas. In lineage exclusion, the conventional
strategy is modified as a phylogenetic pathotype exclusion. Lineage exclusion presumes
that lineage-specific avirulences represent
an evolutionary genetic barrier to pathotype
diversification within the lineage. IRAT212/
N22, RR18-3/Bala, Bala/Tetep, Azucena/Gaurav and several lines from the natural cross of
CR314-5-10 were resistant to leaf blast disease
(CRRI, Annual Report 2000–2001). A combination of genes is also considered useful to
confer resistance to the pathogen lineages
prevalent in China, the USA and Latin America (Babujee and Gnanamanickam, 2000).
Molecular Genetic Analysis
of the Pathogen
Plant disease resistance (R) genes confer
resistance to a wide range of pathogens (fungi,
viruses, bacteria and nematodes); they share
various conserved motifs, suggesting the
existence of a common defence signal transduction pathway in different plant–microbe
interaction systems (Dang and Jones, 2001;
Martin et al., 2003). In general, the R genes
fall into six distinct classes, the most prevalent of which is the nucleotide-binding site
plus leucine-rich repeat (NBS–LRR) genes
(Martin et al., 2003; Qu et al., 2006). The
LRR domains are generally thought to be
involved in the interaction with avirulence
(AVR) proteins and to be the major determinant of resistance specificity (Hulbert
et al., 2001). The AVR-Pita avirulence gene
family has been cloned recently at Kansas
State University, USA, by Khang et al. (2008).
They have studied isolates of the M. grisea
species complex from diverse hosts and have
found that AVR-Pita is a member of a gene
family, which led them to rename it AVRPita1. Using the dominant DNA markers
derived from portions of the Pi-ta gene, 141
rice germplasm accessions were rapidly
determined and the results were confirmed
by inoculating rice germplasm with an M.
grisea strain containing AVR-Pita (Wang
et al., 2007). The Pi-ta gene was found in
accessions from major rice-producing countries, including China, Japan, Vietnam, the
Philippines, Iran and the USA.
Sustainable Management of Rice Blast
In another recent study, Shao et al.
(2008) have reported that the expression of
a hairpin-encoding gene (hrf1), derived
from Xanthomonas oryzae pv. oryzae, confers non-specific resistance in rice to the
blast fungus, M. grisea. Transgenic plants
and their T1–T7 progenies were highly resistant to all major M. grisea races in rice-growing
areas along the Yangtze River, China. The
expression of defence-related genes was activated in resistant transgenic plants and the
formation of melanized appressoria, which
is essential for foliar infection, was inhibited on plant leaves. These results suggest
that hairpins may offer new opportunities
for generating broad-spectrum disease resistance in other crops. However, occurrence of
clustered multigene families is a major
obstacle in the cloning of R genes (Dixon
et al., 1996; Ori et al., 1997), which makes it
even more difficult to determine the functional copy of these genes. Therefore, fine
mapping of R-gene analogues on different
chromosomes would be helpful in the identification of multigene families in rice, which
in turn will lead to the establishment of correlation between the chromosomal position of
known R genes and their analogues. Recently,
Kumar et al. (2007) cloned and also carried
out in silico mapping of resistance gene analogues (RGAs) isolated from rice lines containing known genes for blast resistance. They
have amplified RGAs from the genomic DNA
of 10 rice lines having varying degrees of
resistance to M. grisea by using degenerate
primers. Twenty RGAs were mapped near
to the chromosomal regions containing
known genes for rice blast, bacterial leaf
blight and sheath blight resistance. Thirtynine RGA sequences also contained an open
reading frame representing the signature of
potential disease-resistance genes.
Control Measures
Kato (2001) suggests burning and composting
of infected plant parts; use of non-infected or
certified healthy seeds and disease-resistant
cultivars; appropriate regulation of fertilizer
application; proper cultural control and
99
avoiding damp or most soil with high moisture content for seed sowing, etc. However,
chemical control is the most commonly
used approach in most parts of the globe for
effective disease control. Several fungicides
are used against blast disease, including
benomyl, fthalide, edifenphos, iprobenfos,
tricyclazole, isoprothiolane, probenazole,
pyroquilon, felimzone (= meferimzone),
diclocymet, carpropamid, fenoxanil and
metominostrobin, and antibiotics such as
blasticidin and kasugamycin (Kato, 2001).
The composition, quantity, time and application method of fungicides applied in field
trials are dependent on the disease forecast
for a particular region or zone, or on the
local disease prevalence rate (Kato, 2001).
Carbendazim, chlorobenthiozone, coratop, fungorene, hinosan and kitazin fungicides and antibiotic kasumin were effective
against foliar and neck blast in India (CRRI
Annual Report, 2001–2002). Rice seed treatment with Carbendazim + TMTD 25 was
effective in controlling seedborne blast (CRRI
Annual Report, 2001–2002). The control of
rice blast relies on the use of resistant cultivars and the application of fungicides, but
neither approach is particularly effective in
different geographic locations (Shao et al.,
2008) because management of rice blast via
breeding BRV has had only short-term success due to the frequent breakdown of resistance under field conditions (Valent and
Chumley, 1994). The frequent appearance of
new races (or pathotypes) of the fungus that
are capable of infecting previously resistant
varieties has been proposed as the principal
cause for the loss of resistance (Ou, 1980).
Host resistance in rice to M. grisea functions via a classical gene-for-gene interaction
in which a single dominant resistance gene
corresponds with a dominant avirulence
gene in the pathogen (Hammond-Kosack and
Jones, 1997; Talbot, 2003). Because of the
apparent instability in the genome of M.
grisea, new pathogenic races evolve rapidly
and thus host resistance typically lasts for a
few years only (Zhu et al., 2000; Talbot,
2003). Few fungicides are available for the
effective control of rice blast, but rapid mutation in the pathogen leads to the emergence
of fungicide-resistant variants (Takagaki
100
S. Nandy et al.
et al., 2004); thus, higher-dose applications
of fungicides pose risks both to humans and
the environment.
have been used for rice blast management
for the past 50 years.
Conclusions
Sustainable Rice Blast
Disease Management
In developing countries, poor farmers cannot
afford to control blast disease by the application of fungicides. Chemical control of plant
pathogens is most effective and yet the use of
chemicals is not generally desired due to the
serious environmental threat it poses. Environmental effects and resistance are not
considered a major concern in developing
countries. Farmers are more interested in
short-term strategy for disease control. However, the continuous use of fungicides leads
to the resurgence of resistant races of the
pathogen under selection pressure. Therefore,
sustainable rice blast disease management is
more important for environmental concern.
Figure 8.1 shows the basic components that
We have reviewed here the past 50 years of
research progress in the genetics and molecular biology of rice blast disease, but different approaches can be taken for sustainable
disease control with recent advances in
genomics, proteomics and diverse genetic
resistance mechanisms. Liu et al. (2002)
recently reported the application of candidate defence genes to develop blast-resistant
breeding lines with resistance to diverse
pathogen populations. Several biocontrol
agents for blast have been deployed successfully to combat the disease in the laboratory, greenhouse and field tests (Chatterjee
et al., 1996; Krishnamurthy et al., 1998;
Gnanamanickam et al., 1999). The feasibility of such strategies on a commercial scale
still remains to be tested. Hence, use of
Using
the genetic
diversity of
disease
resistance
Finding
the right gene
Sustainable rice
blast disease
management
Understanding
the physiology
of disease
resistance
Molecular
genetic
analysis of
the pathogen
Fig. 8.1. The four basic components of sustainable rice blast management.
Sustainable Management of Rice Blast
resistant cultivars is the best available alternative to overcome severe yield losses.
The objective of the green revolution
has not changed; there is the added impetus
that crop protection should be conducted in
the context of improving the livelihood of
rural people and preserving limited natural
resources (Leung et al., 2003). However, the
gene revolution has opened up newer and
better possible ways of preventing yield loss
from pathogen attack, conservation and utilization of wild species for resistance genes.
The variability of the pathogen and the history of resistance breakdown have led to the
development of a number of different plant
breeding and molecular approaches to
achieve durable blast resistance. Combinations of resistance genes are thought to provide broader spectra of resistance through
both ordinary gene action and quantitative
101
complementation that result in durable
resistance. Gene pyramiding is one of the
strategies recommended to increase the
durability of blast disease resistance (Robinson, 1973; Nelson, 1978; Buddenhagen, 1983;
Pedersen and Leath, 1988).
Pyramided resistance will be durable in
places where compatibility to the component resistance genes is distributed among
the prevalent lineages. Agricultural practices
such as soil preparation, low nitrogen fertilization, low sowing density, optimized use
of water and seed selection contribute to
reduce the virulence of M. grisea populations. Optimized integration of genetic resistance in agricultural management is the
preferred strategy to protect cultivated rice
from RBD in a way that is affordable, feasible, durable and, overall, compatible with
environmental protection.
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Part III
Biological Control Mechanisms
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9
Postharvest Technology – Yeast
as Biocontrol Agents: Progress,
Problems and Prospects
Neeta Sharma and Pallavi Awasthi
Mycology and Plant Pathology Division, Department of Botany,
University of Lucknow, Lucknow, India
Abstract
Storage losses of fruits in India are high owing to temperature and humidity conditions. Losses in
fruits are estimated to vary between 20 and 30%, valued at nearly 8000 crores annually, depending on
the fruit variety and the postharvest handling system. The application of fungicides to fruits after harvest to reduce decay has been increasingly curtailed due to the development of resistance in pathogens
to many key fungicides, lack of replacement with better fungicides, negative public perception regarding
the safety of pesticides and consequent restrictions on fungicide use. Biological control of postharvest
diseases has emerged as an effective alternative and several products are available in the market. One of
the major limitations with biological disease control is inconsistency in the efficacy of the product. The
limitations of biocontrol products can be addressed by enhancing biocontrol through genetic and environmental manipulations and integration with other alternative methods that, alone, do not provide
adequate protection but, in combination with biocontrol, provide additive or synergistic effects.
Introduction
Approximately half of the population in the
Third World does not have access to adequate food supplies. There are many reasons
for this, one of which is food losses occurring
in the postharvest and marketing system. A
study on ‘Postharvest Food Losses in Developing Countries’ conducted by a committee
of the US National Research Council concludes that, ‘postharvest losses are “enormous”’. The committee extrapolated from
apparent loss patterns and expected production trends and projected postharvest food
losses to be, at a minimum, 47,000,000 Mt
of durable crops and 60,000,000 Mt of perishable crops. ‘The average minimum losses
reported for roots and tubers and fruits and
vegetables were 16 per cent and 21 per cent,
respectively; many more “qualitative” references, not included here, indicate estimates
of 40–50 per cent and above.’
The application of effective fungicides
just prior to or shortly after harvest generally controls postharvest decay (Eckert
and Ogawa, 1988). About 23m kg of fungicides is applied to fruits and vegetables
annually and it is generally accepted that
production and marketing would not be
possible without their use (Ragsdale and
Sisler, 1994). However, use of fungicides
has been restricted due to their carcinogenicity, teratogenicity, residual toxicity and
long degradation period causing environmental pollution (Unnikrishnan and Nath,
2002).
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
109
110
N. Sharma and P. Awasthi
The Food Quality Protection Act (FQPA)
in the USA, the Food and Environment Protection Act (FEPA) 1985 and Control of
Substances Hazardous to Health (COSHH)
Regulations 1988, made under the Health
and Safety at Work Act, 1974, in the UK are
the guiding forces in the regulation of pesticide use in their respective countries. Several countries have implemented their own
specific policies to reduce pesticide use
(Matteson, 1995). Similarly, the fruit industry worldwide has accepted the concept of
integrated fruit production (IFP). IFP aims to
produce high-quality fruit in harmony with
the consumer and the environment. This
implies minimum usage of chemicals, especially after harvest. Globally, greater restrictions on pesticide use in the developed
nations have resulted in increasing trends for
natural, non-chemical or organic approaches
to disease control. Understandably, alternatives to chemical pesticides or products that
allow reduced usage in terms of fewer or
reduced rates of application are beginning to
appear on the market in the form of biological control agents (BCA). The present chapter reviews the status of yeast as a biocontrol
agent and the problems associated with its
commercialization and registration.
Cook and Baker (1983), in their book on
biological control, cited only one example
of the biocontrol of postharvest disease of
strawberry fruit rot using Trichoderma sp.
Subsequently, Wilson and Pusey (1985)
presented their initial research on Bacillus
subtilis to control brown rot on peaches,
caused by Monilinia fructicola, and the
organism was patented. A number of microorganisms (bacteria, yeasts and fungi), which
effectively control postharvest pathogens,
have been identified for the control of postharvest diseases and some of these have been
patented and registered (El-Ghaouth and
Wilson, 1997, 2002). In several studies, yeast
strains (Aureobasidium pullulans, Candida
oleophila, C. guilliermondii, C. sake, Cryptococcus laurentii, Debaryomyces hansenii,
Metschnikowia pulcherrima, Pichia gulliermondii, Sporobolomyces roseus) are reported
for biocontrol of postharvest fungal decays
of fruits caused by Alternaria alternata,
Botrytis cinerea, Geotrichum candidum,
M. fructicola, Penicillium digitatum, P. italicum, P. expansum and Rhizopus stolonifer
(Droby et al., 1989; Wisniewski et al., 1991;
Sharma, 1992, 1993, 2000; Mehrotra et al.,
1996, 1998; Sharma et al., 1997; Spadaro
et al., 2002) . In the past 25 years, research
on biological control of postharvest diseases
has moved from laboratory to practical
applications (Wisniewski and Wilson, 1992;
Wilson and Wisniewski, 1994; Mari and
Guizzardi, 1998; Droby et al., 2001; Janisiewiez and Korsten, 2002; Korsten, 2006).
By early 2000, there were three postharvest biological products available in the
market: Aspire™, a product developed from
C. oleophila (limited to the USA and Israel);
BioSave™, developed from P. syringae to
control decay caused by P. italicum and P.
digitatum (limited to the USA); and YieldPlus™ (limited to South Africa). Avogreen™,
a commercial product of B. subtilis, was
developed to control diseases caused by Cercospora spot and anthracnose of avocado.
Isolation of Antagonist
Often, carposphere, phylloplane, flowers and,
in a few cases, other matrixes have provided
the major source for antagonists (Filonow
et al., 1996; Sharma, 2003; Belve et al., 2006).
Various strategies have been employed to
isolate antagonists and these include isolation from natural cracks on the fruit surface;
agar plates containing apple juice that were
seeded with a rot pathogen (Wilson et al.,
1993); freshly made wounds on apples in
the orchard that were exposed to colonization by fruit-associated microbiota from 1 to
4 weeks before harvest (Janisiewiez, 1996);
and from an apple juice culture resulting
from seeding diluted apple juice with the
orchard-colonized wounds and repeated
reinoculation to fresh apple juice. Isolation
of the antagonists can be improved by using
fruit from unmanaged orchards (Falconi and
Mendgen, 1994) where natural populations
have not been disturbed by chemical usage
and the pool of potential antagonists is
greater than in a chemically managed orchard
(Smolka, 1992).
Postharvest Technology
Natural microflora maintains a balance
among the microbes normally present and
inhibits the growth of newer arrivals. Sharma
(2005) reported that undiluted fruit washings when plated on agar plates exhibited a
dense population of yeast and bacteria and,
on dilution, filamentous fungi of the pathogenic type were isolated. This suggests that
bacteria and yeast, naturally present on the
surface, may inhibit the growth of other
microorganisms, including plant pathogenic
fungi. Later, it was observed that the citrus
fruits, when washed and stored, rotted faster
than the unwashed fruits, suggesting that
these bacteria and yeast provide protection
to fruits against postharvest pathogens.
Rather than in vitro screening of organisms in Petri plates, which favoured the
identification of antibiotic-producing organisms, a selection strategy was developed to
identify suitable yeast antagonists (Wilson
et al., 1993). The method involved placing
washing fluids obtained from the surface of
the fruit into fruit wounds that subsequently
were inoculated with a rot pathogen. Organisms were then isolated from the surface of
wounds that did not develop infections.
These were plated out and isolated.
Pure cultures of potential antagonists
were produced and then each organism was
screened individually to assess its potential
as a biocontrol agent. This method identified a number of antagonists that were studied more intensely and measured against
the criteria set for suitability for commercial
production, as outlined by Wilson and Wisniewski (1989) and Hofstein et al. (1994):
●
●
●
●
●
●
●
●
genetically stable
effective at low concentrations
not fastidious in its nutrient requirements
ability to survive adverse environmental conditions (including low temperature and controlled atmosphere storage)
effective against a wide range of pathogens on a variety of fruits and vegetables
amenable to production on an inexpensive growth medium
amenable to a formulation with a long
shelf life
easy to dispense
●
●
●
●
●
111
does not produce metabolites that are
deleterious to human health
resistant to pesticides
compatible with commercial processing procedures
does not grow at 37°C and is not associated with infections in humans
non-pathogenic to host commodity.
Biocontrol Activity
Most antagonistic yeasts are efficient colonizers, even under adverse environmental
conditions, as they utilize nutrients rapidly,
produce extracellular materials that enhance
their survival on fruit surfaces and restrict
both colonization sites and flow of germination caused to fungal propagules (Dugan
and Roberts, 1995). In order to optimize disease control, it is important to understand
the mode of action of the antagonists so that
these attributes can be utilized to improve
performance. The antagonist activity can be
expressed in a number of ways. The most
common is antibiosis (production of metabolites such as pyrrolnitrin or iturins), attributed
mainly to bacterial antagonists (Smilanick
and Dennis-Arrue, 1992). The antibiotic
pyrrolnitrin, produced by Pseudomonas
cepacia LT-4-12W (Janisiewiez and Roitman, 1988), reduced in vitro growth and
conidia germination and controlled the
pome fruit pathogens, P. expansum and B.
cinerea, and citrus fruit pathogen, P. italicum. However, the significance of the antibiotics in these biocontrol situations was
not clear, since strain LT-4-12W still provided substantial control of blue mould
decay on oranges inoculated with laboratoryderived mutants of P. italicum resistant to
pyrrolnitrin. Spadaro et al. (2002), in studies on M. pulcherrima, found that in the
in vitro antagonism studies on different substrates, the yeast could produce some metabolites toxic to the pathogen, as distinct from
the application of culture filtrates in vivo. In
recent years, the use of antibiotic-producing
bacteria has been abandoned in order to prevent the appearance of resistance in pathogen strains for humans or animals.
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N. Sharma and P. Awasthi
Competition for nutrients and/or space
is the major mechanism involved for P. guilliermondii, C. laurentii, C. utilis, C. oleophila, D. hansenii and several other yeasts
employed as bioagents (Chalutz and Wilson, 1990; Arras, 1996; Arras et al., 1997; Spadaro et al., 2002; He et al., 2003; Chan and
Tian, 2005; Zhang et al., 2005). Janisiewiez
et al. (2000) developed a non-destructive
method using tissue culture plates having a
defusing membrane at the lower end of
cylindrical inserts for in vitro study of competition for nutrients separated from the
competition for space. Living cells of the
antagonist are necessary to guarantee fungal
control. The ability to prevent infection by
pathogen was lost when the antagonist cells
were killed. It was also observed that competition for nutrients was not visible when a
surplus of nutrients was available. Therefore, the nutritional environment available
at the wound site may create a favourable
microenvironment for antagonists to colonize,
multiply and compete effectively (Zheng
et al., 2004). The activity of an antagonist is
dependent on the concentration of the
antagonist: the higher the concentration, the
more effective the control. The antagonist
cell concentration of 106 – 108 CFU/ml or
more of Candida spp., D. hansenii and Pantoea agglomerans provided satisfactory levels of control (Droby et al., 1989; McLaughlin
et al., 1990). However, different isolates of
M. pulcherrima at 106 CFU/ml were not
found to provide satisfactory levels of control against B. cinerea and P. expansum
(Spadaro et al., 2002).
While early studies indicated that nutrient competition and the fast growth rate of
antagonists played a major role in biocontrol activity, subsequent studies indicated a
much more complex interaction, such as
direct interaction with the pathogen (Wisniewski et al., 1991; Spadaro et al., 2002),
induced resistance in host tissue (Wilson
et al., 1994; Droby et al., 2002) or a gamut of
interactions between the antagonist, pathogen and commodity. Pichia guilliermondii
US-7 (Droby et al., 1989) and M. pulcherrima
(Spadaro et al., 2002) exhibited nutrient competition along with direct parasitism against
B. cinerea in apples. Pichia membranefaciens
and C. albidus exhibited tenacious attachment with pathogen hyphae, along with
secretion of extracellular lytic enzymes
(Chan and Tian, 2005). Ultrastructural and
cytochemical studies on yeast, C. saitoana,
when co-cultivated with B. cinerea, showed
cytological damage as papillae and protuberances in the cell wall and degeneration
of the cytoplasm. It was also found to stimulate structural defence response in the
host. Host cell walls were well preserved and
displayed an intense and regular celluloselabelling pattern, as seen in transmission
electron microscopy (El Ghaouth et al.,
1998).
Yeast cells are able to produce hydrolytic enzymes capable of attacking the cell
walls of pathogens and extracellular polymers that appear to have antifungal activity.
Yeast, P. anomala strain K, effective in the
control of grey mould of apple, increased
production of exo-b-1,3-glucanase threefold
in the presence of cell wall preparations of
B. cinerea in apple wounds. Higher b-1,3glucanase and chitinase activity was also
detected in apple wounds treated with
strains of another antagonist, A. pullulans,
effective in controlling various decays on
apple, table grape and other fruits (Ippolito
et al., 2000; Castoria et al., 2001). Yeast, P.
membranefaciens and C. albidus, show
b-1,3-glucanase and exo-chitinase activity
in the presence of cell wall preparations of
R. stolonifer, M. fructicola and P. expansum
(Chan and Tian, 2005).
Yeasts like C. famata are reported to
control green mould due to induction of
phytoalexins, scoparone and scopolectin
(Arras, 1996). However, the role of enzymes
and phytoalexins in biocontrol activity warrants further investigation. Fajardo et al.
(1998) reported differential induction of
proteins in orange flavedo by biologically
based elicitors. More recently, molecular
approaches to examine the mode of action
have been studied on the biocontrol agent. A
transformation system for C. oleophila yeast
produced yeast lines with either higher or
lower levels of a b-1,3-glucanase gene/enzyme
expression compared to the wild type. Biocontrol activity did not differ between the
different yeast lines, but the results did not
Postharvest Technology
rule out a role for this gene in biocontrol
activity. It was also demonstrated that overexpression of a lytic peptide belonging to the
defensin family of antimicrobial peptides in
yeast could enhance biocontrol activity
(Segal et al., 2002; Yehuda et al., 2003).
Constraints in Product
Development and Registration
In the early years, several yeast antagonists
that had commercial potential were misidentified, such as strain US-7 of C. guilliermondi, which was misidentified originally
as D. hansenii. This caused some confusion
in the patenting process and emphasized
the need to have at least two confirming
identifications by reputable yeast taxonomic
services. It also emphasized the weakness of
using physiological tests as the basis for making taxonomic determinations (McLaughlin
et al., 1990). Also, few isolates of C. guillliermondii were abandoned because they were
found to be pathogenic to humans.
Potential biocontrol agents often have
some significant limitations: sensitivity to
adverse environmental conditions such as
extreme dryness, heat and cold, limited
shelf life, limited biocontrol efficacy in situations where several pathogens are involved
in decay development and an inability to
control latent infections. For commercialization, several semi-commercial and commercial trials have to be conducted, for which
large volumes of antagonist are required. The
mass production of the bioagent by rapid,
efficient and inexpensive fermentation of the
antagonist is a key issue. Therefore, it is fundamental to find carbon and nitrogen sources
that provide maximum biomass production
at minimum cost, while maintaining biocontrol efficacy. Cheap industrial waste materials such as cottonseed meal, corn steep liquor,
partially digested peptone, yeast extract, dry
brewer’s yeast, sucrose and molasses have
been used as growth media for the multiplication of cells (Hofstein et al., 1994; Costa
et al., 2001).
Large-scale production of any yeast
depends on the amount of technical information available on that specific strain, such
113
as osmotolerance, temperature, oxygen requirements, optimum pH and optimum growth
rate. Growth rate of yeast is very high, but
lower than that of bacteria; longer fermentation durations pose the risk of yeast cultures becoming contaminated. Yeast is also
sensitive to low pH (below three), which is
used generally as a measure to check bacterial contamination because pH above five
is favourable for bacteria that may contaminate yeast culture. Aeration of fermentors, to fulfil the oxygen requirement for
maximum output, can also be a source of
contamination during the early phases of
production and, to prevent such contamination, other technologies must be used. The
contaminants should be identified at each
stage of production and quantified in the
end product.
Yeast fermentation is an exothermic
process; therefore, the fermentation temperature can never be below ambient and, since
yeasts appear sensitive to high temperatures
(above 28°C), a cooling system more efficient than the evaporative system routinely
used has to be employed. This, however,
adds to the cost of production.
A major obstacle to the commercialization of biocontrol products is the development of a shelf-stable product that retains
bioactivity similar to that of fresh cells. Formulations can influence the survival and
activity of biocontrol agents. An accurate
formulation has a profound effect on the
efficacy of a biocontrol agent, including its
shelf life, ability to grow and survive after
application, effectiveness in disease control, ease of operation and application and
the cost (Fravel et al., 1998). A biofungicide
should be effective for at least 6 months,
and preferably for 2 years (Pusey, 1994).
This can be achieved by supplementing the
yeast with protectants, carriers or additives.
Alternatively, yeast can be conditioned during fermentation by using an emulsifier.
Drying the product and maintenance in
a dry environment or suspension in oil are
common approaches. Products are available
as wettable powder, as frozen cell concentrated pellets or as liquid formulations. It
was found that freeze-dried cells were significantly less effective than fresh cells.
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N. Sharma and P. Awasthi
Certain freeze-drying protective agents and
rehydration media enhanced the viability of
the antagonist, P. agglomerans strain CPA-2,
effective against blue mould and grey mould
of pome fruits (Costa et al., 2000). Survival
of cells of the antagonistic yeast, C. sake,
was improved from 0.2% to 30–40%, by
using freeze-drying protective media consisting of skim milk and other protectants,
such as 10% lactose or glucose and 10%
fructose or sucrose. The presence of trehalose in liquid formulations appeared to help
preserve the viability of C. sake during storage. It is known that intracellular trehalose
exerts a protective effect on yeast under
extreme environmental conditions such as
desiccation, freezing, osmotic stress and
heat shock, and it also provides thermal stability to the cells (Abadias et al., 2001).
The application of adjuvant can protect
and stimulate the establishing of the antagonist on the host surface. The addition of
xanthan gum to A. pullulans L47, applied to
strawberries in the field from bloom to fruit
at the green stage, improved survival of the
antagonist and increased biocontrol of storage rot caused by B. cinerea (Ippolito et al.,
1998). Formulations may include wetters
(humectants) to facilitate reabsorption of
moisture from air. Wetters not only make
water spray stay on plants but, like oil carriers, they also enable organisms to reach otherwise inaccessible places such as depressions,
stomata and lenticels, thereby improving the
chances of establishing antagonists for disease control. Oil carriers are expensive, but
formulations containing oils can enhance
the reliability of biological control agents
(Jones and Burges, 1998). Research is needed
to determine the value of each additive alone
and also in the presence of other ingredients, as well as to ensure the requirements
for ecological safety.
One of the major limitations with biological disease control is the inconsistency
in efficacy that is often observed when useful antagonists reach the stage of large-scale
testing, and which can arise from a variety
of causes reflecting the biological nature of
the control microorganism. Essentially, the
organism must first survive application and
then retain activity in the environment of
use throughout the period when active control is required, which may be several months
for some pathogens. During this time, it
must survive fluctuations in the physical
environment and the action of the indigenous and competitive microbiota. The use
of appropriate inoculum production, formulation and application technologies,
together with quality control checks, should
also help in this process. Nevertheless, even
if reliable BCAs can be produced, they must
still be easy to use and cost-effective or they
will either never reach the marketplace or
not be used by growers.
By early 2000, there were two yeastbased postharvest biological products available on the market: Aspire™ (C. oleophila
I-182) and YieldPlus™ (El-Ghaouth and
Wilson, 1997, 2002; Wilson and El-Ghaouth,
2002). The commercial development of Aspire
by Ecogen-Israel Partnership Ltd, focused on
the biocontrol of postharvest decays of citrus,
mainly blue mould and green mould caused
by P. italicum and P. digitatum, respectively,
which invade through wounds after harvest. Throughout the course of developing
Aspire™, considerable research went into
finding methods to enhance the reliability
and efficacy of the product and other
selected antagonists as well.
As a result, second generation biocontrol products were developed using a combination of natural products along with a
yeast antagonist to address the poor ability.
Research efforts led to the development of
two new products whose main components
consisted of the yeast antagonist, C. saitoana,
and either a derivative of chitosan (Biocoat)
or lysozyme (Biocure) (El Ghaouth et al.,
2000a). Both compounds have been tested
worldwide and have shown strong eradicant
activity. Both products contain additional
additives, such as sodium bicarbonate, to
enhance efficacy and perform as well as the
postharvest fungicides currently available.
Another constraint concerns registration. Currently, there are no fungal biocontrol products registered and sold worldwide.
Some products are available in several countries, while others are sold in their respective
countries. This reflects the problems associated with registration requirements in
Postharvest Technology
different countries and includes concerns
about releasing non-indigenous microorganisms. The legislation drafted essentially for
chemical pesticides is not always applicable
to biological pesticides and the requirements for the registration of biological pesticides are currently under discussion for
appropriate review.
The position of the biocontrol product
in the market governs its future. For example, if the product enters the agrochemical
market, it competes against synthetic fungicides that can kill pathogenic organisms,
while yeast only-based products cannot do
so and neither do they have systemic action.
They act mainly as protectants that may also
induce resistance in the hosts. The other
option is to position the product in the ‘all
green’ category in markets such as those of
perishables, where no other option is available, thus eliminating any competition and
fulfilling the principal objective of consumer and environmental safety.
Integrated Control
Since, biological agents alone are not capable of providing commercially acceptable
levels of control, their integration with other
control measures is expected to provide
greater stability and effectiveness. It is also
desirable that the use of antagonists must be
compatible with current handling and storage practices which could otherwise cause a
reduction in the effectiveness of antagonist
strains. For biological control to be effective,
use of antagonists must be compatible with
other control measures. An effective biocontrol based on a mixture of several complementary and non-competitive antagonists
has several advantages: apart from a wider
spectrum of activity, they increase efficacy,
are more reliable and allow reduction in
application times and treatment costs. They
also permit the combination of different
genetic characteristics, minimizing the need
for genetic engineering. In a study on apples,
a broader spectrum of pathogens was controlled and less total biomass of the antagonist was needed to control decay (Janisiewiez,
115
1996) when a mixture of antagonists was
applied. The mixtures are either paired at
random or after screening, for minimum
mutual niche overlap. To determine further
compatibility of the strains selected, it is
important to conduct coexistence studies
using De Wit displacement series in fruit
wounds (Wilson and Lindow, 1994). The
benefits of this approach are clear, but its
implementation requires approval from the
industry. It also entails doubling of the cost.
However, this can be overcome by using in
the mixture at least one antagonist which
has been commercialized.
Some exogenous substances, such as
chitosan, amino acids, antibiotics, calcium
salts and carbohydrates, have been studied
to enhance the biocontrol capability of
antagonists against fungal pathogens. Calcium chloride improved biological control
of the yeast, P. guilliermondii (Droby et al.,
1997). Combining 0.2% glycolchitosan with
the antagonist, C. saitoana, was more effective in controlling green mould of oranges
and lemons, caused by P. digitatum, and
grey and blue moulds of apples than either
treatment alone (El- Ghaouth et al., 2000a,b).
In a recent study by the authors, a combination of chitosan and the yeast, C. utilis, was
found effective in controlling postharvest
pathogens on tomato (Sharma et al., 2006).
The studies also showed that several yeast
genera were compatible with low concentrations of chitosan and the protection
afforded by this combination was superior
to the stand-alone treatments.
GRAS (generally recognized as safe) substances such as sodium carbonate, sodium
bicarbonate and ethanol reduced conidial
germination of P. digitatum, the causal
agent of green mould of citrus. Ethanol at
10%, in combination with ethanol-resistant
S. cerevisiae strains 1440 and 1749, reduced
the incidence of grey mould decay on apples
from more than 90% to close to 0%, respectively, whereas either treatment alone did
not reduce decay. The same concentration
of ethanol reduced green mould of lemons to
less than 5% (Smilanick et al., 1995, 1999).
A. pullulans, in combination with calcium chloride or sodium bicarbonate, was
found effective in controlling postharvest
116
N. Sharma and P. Awasthi
pathogens on sweet cherries (Ippolito et al.,
1998).
Pre-storage hot air treatment of apples
reduced or eliminated blue mould decay
caused by P. expansum and grey mould
decay (Fallik et al., 1995). Heat also
improved biocontrol with heat-tolerant
yeasts when applied to apples up to 24 h
after inoculation with the pathogen. The
heat treatment alone provided little residual protection, but the residual protection
provided by Ca and the antagonist in combination enhanced the control by heat. When
antagonists were applied to apple wounds
before heat treatment, the heat reduced
populations of P. syringae and increased
populations of the two heat-tolerant yeasts
more than tenfold.
Conclusions
Future lines of research should be directed
to find methods of enhancing the reliability
and efficacy of selected antagonists, and the
field is gaining momentum. It should aim at
finding additives or physical control methods that will act synergistically with the
antagonist. This involves combining the
product with a low-level of postharvest fungicide or GRAS substances. It has been
reported that physical treatments such as hot
air, curing, hot water brushing and combinations of the above with pressure infiltration
of calcium could also increase the efficacy of
antagonists. Using mixtures of antagonists,
or combining antagonists with specific nutrients or sugar analogues, is also suggested as
an approach to increase efficacy.
Genetic manipulation of antagonists is
a field in its infancy. Current efforts are
focused on developing efficient transformation procedures for yeast antagonists and
inserting genes for tracking the antagonist
in the environment rather than enhancing
biocontrol (Yehuda et al., 2001).
Other approaches could be: the insertion of the gene for amylase under the constitutive promoter in some BCAs to allow
effective use of the fruit carposphere starch;
biocontrol strains with a higher capability
to exploit the nitrogen compounds present
or with a higher transport or metabolism
rate of the limiting factor can be developed,
because nitrogen is often a limiting substance when the biocontrol mechanism of
action is competition for nutrients; and use
of mutants that use new substrates, not
metabolized by the pathogen, to provide a
nutritional advantage or attempt to obtain
strains resistant to phenolic compounds
(Bizeau et al., 1989).
Early experiments in transformation
for marker genes have been successful.
Metschnikowia pulcherrima was transformed with the green fluorescent protein
gene (Nigro et al., 1999) and histidine
auxotrophs of C. oleophila were transformed with HIS3, HIS4 and HIS5 genes
(Chand-Goyal et al., 1999). In all cases, the
transformed antagonists maintained their
biocontrol capability and there were no
detectable differences between the wild type
and the transformants. All these studies were
accomplished only to obtain variants of the
antagonistic strains with a genetically stable
marker. Jones and Prusky (2002) investigated the possibility of expressing a DNA
sequence in S. cerevisiae to allow the production of a cecropin A-based antifungal
peptide. Yeast transformants inhibited the
growth of germinated Colletotrichum coccoides spores and inhibited decay developments caused by the pathogen in tomato
fruit. The lack of activity toward non-target
organisms by the peptide and the use of S.
cerevisiae as a delivery system suggest that
this method could provide a safe alternative for postharvest disease control. However, attempts to overexpress genes involved
in biocontrol, for example, lytic enzymes,
or engineering strains with desired biocontrol traits may soon yield positive results.
Biological control of plant diseases in general and on fruit after harvest in particular
is a niche market, with a relatively small
profit potential. However, it is clear that the
stage is set for biological control agents to
play a greater part in agriculture and horticulture. This approach undoubtedly would
encourage environmentally desirable products that are desired by the public to reach
the marketplace rapidly.
Postharvest Technology
117
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10
Biological Control of Plant Diseases:
An Overview and the Trichoderma
System as Biocontrol Agents
Abhishek Tripathi,1 Neeta Sharma2 and Nidhi Tripathi1
1Department
of Bioscience and Biotechnology, Banasthali University,
Banasthali Vidyapith, India; 2Mycology and Plant Pathology Division,
Department of Botany, University of Lucknow, Lucknow, India
Abstract
Biocontrol is the reduction of inoculum density or disease-producing activities of a pathogen in its
active or dormant state by one or more microorganisms, accomplished naturally. Research on biological control agents has utilized naturally occurring saprophytic soil fungi to compete with and/or
destroy soilborne pathogens. Biological control has attracted attention from researchers for over 30 years,
primarily because of the interest in developing more ‘environmentally friendly’ means of disease management in the absence of agricultural pesticides. Despite considerable effort in the area of biological control,
few practical applications have become established in agriculture for the control of plant diseases. Common biocontrol agents include Trichoderma, Gliocladium, Aspergillus, Penicillium, Chaetomium, Dactylella, Glomus, etc. Biological control is achieved by competition, hyperparasitism, induced resisitance,
hypovirulence, etc. Mycoparasitism and production of volatile and non-volatile antibiotics are important
mechanisms operating in the case of Trichoderma, besides commercial uses and mass multiplication of
the novel biocontrol agent. The future of biocontrol lies perhaps with the development of better application methods and the use of genetic engineering to increase the efficacy of various wild strains.
Introduction
Empirical approaches to chemical disease
control have been practised since ancient
times, when concoctions consisting of salt
brine, sulphur, lime, ashes and salts of copper, mercury and arsenic were used to combat disease. Reports of pesticide residues in
food, soil, river and groundwater undermine
consumers’ trust. Thus, the increasing
concern, particularly in developed nations,
is that modern methods of crop protection
have an overall negative impact on the environment and on society. Pathogen resistance
against certain classes of fungicides has further reduced the number of disease control
measures available. In recent years, it has
become evident, as a result of public opinion and environmental laws, that new and
safer alternatives to traditional synthetic
pesticides are both desirable and mandated.
Research emphasis has therefore been on
the development of alternative approaches
to control the pathogens and pests of ornamental crops using biocontrol agents.
There are considerably more success stories involving the control of insect pests. Garrett’s (1965) definition of biological control
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
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A. Tripathi et al.
of plant disease was, ‘any condition under
which or practice whereby survival or activity of a pathogen is reduced through the
agency of another living organism (except
man himself), with the result that there is a
reduction in the incidence of the disease
caused by the pathogen’. Although biological control consists of diverse methods and
approaches to suppress plant disease, in most
cases antagonists to pathogens are added to
the agroecosystem. Various approaches of
biocontrol are directed at suppressing initial disease induced by a soilborne pathogen
or the application of an avirulent isolate of
the pathogen that ‘competes’ with the virulent pathogen on or in the host. Biological
control employs living agents (usually antagonists or competitors of the causal agent) to
control plant diseases. Effective biological
controls take advantage of the natural competition of living organisms for limited resources
or ecological niches. Thus, two organisms
cannot occupy the same space at the same
time, they cannot consume the same resource
(e.g. food source) at the same time and, in
some cases, one organism produces compounds that are inhibitory to the growth
and development of the other organism.
Certain microorganisms that normally compete for and live off debris and dead animal
and plant cells in the soil environment have
developed, through mutation, the ability to
invade a host plant and escape the effects of
antagonists. These invading organisms are
referred to as pathogens. The lack of survival of the pathogen and the superior competitiveness of the antagonists relative to
pathogens brings promise to the theory of
using antagonists to control pathogens.
Tubeuf (1914) coined the term ‘biological control’ in relation to plant pathogens,
while Hartley (1921) first attempted to control the root diseases of plants with introduced microorganisms. Cook and Baker
(1983) defined biological control as, ‘the
reductions of the amount of inoculum or
disease-producing activity of a pathogen
accomplished by one or more organisms
other than man’. Microorganisms, which are
used in the management of plant diseases,
are referred to as ‘biocontrol agents’. The
important genera of fungi used as biocontrol
agents are Trichoderma, Gliocladium, Aspergillus, Penicillium, Neurospora, Chaetomium,
Dactylella, Arthrobotrys and Glomus, etc.
According to Baker (1987), biological
control is the decrease of pathogen activity
accomplished by one or more organisms
including the host plant but excluding
humans. Harman (2000) defined biological
control as a critically needed component of
plant disease management. Biocontrol agents
are known as antagonists. The most important, well-studied antagonists against several
plant pathogens are fungi like Ampelomyces sp., Aspergillus spp. (particularly A.
niger and A. terreus), Chaetomium globosum, Coniothyrium minitans, Fusarium sp.,
Gliocladium virens, Penicillium citrinum,
Peniophora gigantea, Trichoderma spp.
(particularly T. harzianum and T. viride)
and Sporodesmium sp.; and bacteria like
Agrobacterium radiobacter strain K84, species of Bacillus, Enterobacter, Micromonospora, Pseudomonas and Streptomyces.
Mechanisms of Biological
Control of Plant Diseases
Competition
Competition occurs between microorganisms when space or nutrients (i.e. carbon,
nitrogen and iron) are limiting and its role
in the biocontrol of plant pathogens has
been studied for many years, with special
emphasis on bacterial biocontrol agents. An
important attribute of a successful rhizosphere biocontrol agent would be the ability
to remain at a high population density on
the root surface, providing protection of the
whole root for the duration of its life. Mycorrhizal fungi can also be considered to act
as a sophisticated form of competition or
cross-protection, decreasing the incidence
of root disease. Fomes (Heterobasidion)
annosum colonizes stumps of freshly cut
pine and other conifers and spreads via root
grafts to other healthy trees, where it causes
root rot (refer to Chapter 26). Spraying
freshly cut stumps with spore suspensions
of Phlebia (Peniophora) gigantea will prevent
Biological Control of Plant Diseases
H. annosum from getting a foothold, and
this is standard practice in the UK.
Antibiosis
Antibiosis is the inhibition of an organism
by a metabolic product (such as an antibiotic)
from another organism. Many organisms,
especially soil fungi and Actinomycetes,
produce antibiotic substances. The production of antibiotics by Actinomycetes, bacteria and fungi is demonstrated very simply
in vivo. Numerous agar plate tests have been
developed to detect volatile and non-volatile antibiotic products by putative biocontrol agents and to quantify their effects on
pathogens. In general, however, the role of
antibiotic production in biological control
in vitro remains unproved. Three diseases
can be controlled by antibiosis: Armillaria
root rot by T. viride, Pythium and Rhizoctonia damping off and stem and root rot diseases by P. fluorescens and crown gall by A.
radiobacter. The most widely accepted
commercial example is the control of crown
gall using strain 84.
Hyperparasitism and mycoparasitism
Biological control can occur through direct
parasitism. Parasitism involves the direct utilization of food of one organism by another
organism. Hyperparasites are organisms parasitic on other parasites. Some have referred
to this as ‘natural biocontrol’. A few examples of hyperparasitism include: Darluca
(Sphaerellopsis) filum parasitizes rust fungi
and species of Ampelomyces parasitize powdery mildews; Tuberculina maxima parasitizes the aecial stage of Cronartium ribicola,
cause of white pine blister rust; T. viride,
and a number of other species, are known to
parasitize hyphae of R. solani. The most
common example of mycoparasitism is that
of Trichoderma sp., which attack a great
variety of phytopathogenic fungi responsible for the most important diseases suffered
by crops of major economic importance
worldwide.
123
Hypovirulence
Hypovirulence is a term used to describe
reduced virulence found in some strains of
pathogens. This phenomenon was first
observed in Cryphonectria (Endothia) parasitica (chestnut blight fungus) on European
Castanea sativa in Italy, where naturally
occurring hypovirulent strains were able to
reduce the effect of virulent ones. These
slower-growing hypovirulent strains contain
a single cytoplasmic element of doublestranded RNA (dsRNA) similar to that found
in mycoviruses, which is transmitted by anastomosis in compatible strains through natural
virulent populations of C. parasitica. Hypovirulence has also been reported in many
other pathogens, including R. solani, Gaeumannomyces graminis var. tritici and Ophiostoma ulmi, but the transmissible elements
responsible for hypovirulence or reduced
vigour of the fungi are subject to debate and
may be due to dsRNAs, plasmids or viruses.
Induced Resistance and
Cross-Protection
Induced resistance is a plant response to
challenge by microorganisms or abiotic
agents such that, following the inducing
challenge, de novo resistance to pathogens
is shown in normally susceptible plants.
Both localized and systemic-induced resistance are non-specific and can act against a
whole range of pathogens, but whereas
localized resistance occurs in many plant
species, systemic resistance is limited to
some plants. Cross-protection differs from
induced resistance in that, following inoculation with avirulent strains of pathogens or
other microorganisms, both inducing microorganisms and challenge pathogens occur
on or within the protected tissue. The most
commonly reported examples of crossprotection involving fungi are probably
those used against vascular wilts. Inoculation with non-pathogenic formae speciales
of Fusarium and Verticillium species, or
with other fungi or bacteria, has shown different levels of cross-protection.
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A. Tripathi et al.
Predation
Predation has also been examined as a potential form of biocontrol. Nematode-trapping
fungi and predaceous nematodes have been
studied in detail as potential biological control agents, but ultimately have had little
effect on the numbers of plant parasitic
nematodes in the soil.
Mycorrhizae
Mycorrhizae are symbiotic (mutualistic)
associations between fungi and plant roots.
The increased surface area provided by
mycorrhizal fungi allows for increased
nutrient uptake, which indirectly benefits
disease management derived by healthier,
more vigorous roots. Because of the generally beneficial effect of mycorrhizae on plant
growth and their common occurrence, many
investigations have looked into the potential of root–fungus associations as potential
biological control agents.
Vesicular arbuscular mycorrhizal (VAM)
fungi were recognized and described in the
last few decades of the 19th century. The
term ‘VAM’ was changed to ‘AM’ by Draft and
Nicolson (1974) because some species did not
form vesicles. AM fungi occur throughout
the terrestrial ecosystem in almost all the
herbaceous and woody plants, forming a
symbiotic relationship with the roots (Gerdemann, 1968; Trappe and Fogel, 1977). This
symbiotic association has been reported to
play an important role in plant mineral nutrition (Gianinazzi and Gianinazzi-Pearson,
1986). It has been observed by several workers that these fungi facilitate the uptake of
many nutrients (phosphorus, zinc, copper,
sulphur, potassium, iron, calcium, etc.),
resulting in increased biomass (Wani and
Lee, 1992). Nutrient content of N, P and K,
and also Fe, Mn and Cu, increased due to
AM inoculation in papaya. Among all the
AM species, G. mosseae was recorded as the
most efficient for nutrient uptake. Rajeshwari et al. (2001) reported that G. fasciculatum at low phosphorus level increased the
root and shoot biomass. They also recorded
that AM colonization was reduced at higher
phosphorus level.
Biocontrol of Airborne Diseases
Many naturally occurring microorganisms
have been used to control diseases on the aerial surfaces of plants. The most common
bacterial species that have been used for the
control of diseases in the phylloshpere include
P. syringae, P. fluorescens, P. cepacia, Erwinia
herbicola and B. subtilis. Fungal genera that
have been used for the control of airborne
diseases include T. ampelomyces and the
yeasts, Tilletiopsis and Sporobolomyces.
Biocontrol agents normally must achieve
a high population in the phyllosphere to
control other strains, but colonization by the
agent may be reduced by competition with
the indigenous microflora. Integration of
chemical pesticides and biocontrol agents
has been reported with Trichoderma spp.
and P. syringae. Biocontrol agents tolerant
to specific pesticides could be constructed
using molecular techniques. Resistance to
the fungicide benomyl is conferred by a single amino acid substitution in one of the
b-tubulins of T. viride, the corresponding
gene thereby producing a biological control
agent that could be applied simultaneously
or in alternation with the fungicide.
Biocontrol of Soilborne Diseases
Chemical control of soilborne plant diseases
is frequently ineffective because of the
physical and chemical heterogeneity of the
soil, which may prevent effective concentration of the chemicals from reaching the
pathogen. Biological control agents colonize
the rhizosphere, the site requiring protection and leave no toxic residues, as opposed
to chemicals. Microorganisms have been
used extensively for the biological control of
soilborne plant diseases, as well as for promoting plant growth. Fluorescent Pseudomonads are the most frequently used bacteria
for biological control and plant growth
promotion, but Bacillus and Streptomyces
Biological Control of Plant Diseases
species have also been commonly used.
Trichoderma, Gliocadium and Coniothyrium
are the most commonly used fungal biocontrol agents. Perhaps the most successful
biocontrol agent of a soilborne pathogen is
A. radiobactor strain K84, used against crown
gall disease caused by A. tumefaciens.
Molecular techniques have also facilitated the introduction of beneficial traits
into rhizosphere competent organisms to
produce potential biocontrol agents. Chitin
and b-(1,3)-glucan are the two major structural components of many plant pathogenic
fungi, except Oomycetes, which contain cellulose in their cell wall and no appreciable
levels of chitin. Biological control of some
soilborne fungal diseases has been correlated
with chitinase production. Bacteria producing chitinases or glucanases exhibit antagonism in vitro against fungi. A recombinant
Escherichia coli expressing the chiA gene
from S. marcescens was effective in reducing disease incidence caused by Screrotium
rolfsii and R. solani. In other studies, chitinase genes from S. marcescens have been
125
expressed in Pseudomonas spp. and the
plant symbiont, Rhizobium meliloti. The
modified Pseudomonas strain controlled
the pathogens, F. oxysporum f. sp. rodelens
and G. graminis var. tritici.
Commercial Biocontrol Agents
The following is a list of commercially available products formulated for the biocontrol
of plant pathogens and/or plant growth promotion involving the induction of plant host
defence. The list originated in 2000 through
the efforts of Dr Deborah Fravel, USDA-ARS,
and is now being updated by the APS Biological Control Committee (Table 10.1).
The Trichoderma System as
Biocontrol Agents
Trichoderma spp. are free-living, saprophytic
fungi that exhibit a high rate of interactions
Table 10.1. Fungi, bacteria, activators and their available commercial products.
Commercial products
Fungi
Ampelomyces quisqualis
Candida oleophila
Coniothyrium minitans
Fusarium oxysporum
Gliocladium sp.
Myrothecium verrucaria
Paecilomyces lilacinus
Phlebia gigantea
Pythium oligandrum
Trichoderma sp.
Bacteria
Agrobacterium radiobacter
Bacillus sp.
Burkholderia cepacia
Pseudomonas sp.
Streptomyces sp.
Activators of host defence
Bacteria
Bacterial protein
Synthetic chemical
AQ10
Aspire
Contans, Intercept WG, KONI
Biofox C, Fusaclean
Primastorp, SoilGard
DiTera
Paecil
Rotstop
Polyversum
Bio Fungus, Binab T, Root Pro, RootShield/PlantShield, T-22G,
T-22 Planter Box, Trichodex, Trichopel, Trieco
Galltrol, Nogall
BioYield, Companion, EcoGuard, HiStick N/T, Kodiak, Rhizo Plus,
Serenade, Subtilex, YieldShield
Deny, Intercept
BioJect Spot-Less, Bio-save, BlightBan, Cedomon
Actinovate, Mycostop
Actinovate, BioYield, YieldShield
Messenger
Actigard
126
A. Tripathi et al.
with root, soil and foliar environments. The
antagonistic nature of fungi from the genus
Trichoderma was demonstrated more than
70 years ago. Furthermore, excellent progress
has been made towards the improvement of
Trichoderma sp. as a biological control
agent in the past few years. Many Trichoderma isolates have been used as biocontrol
agents against soilborne pathogens (Weindling, 1934). Trichoderma is a ubiquitous
genus present in almost all types of habitat
fungal antagonists. It comprises 3% of the
total fungal population in forests and 1.5%
of the total fungal population in other soils.
It also exhibits the property of competition
with fellow plant pathogenic fungi for key
exudates from seeds that stimulate the germination of propagules of plant pathogenic fungi
in soil, and also with soil microorganisms for
nutrients and space. Trichoderma spp. act
against a range of economically important
aerial and soilborne plant pathogens. They
have been used in the field and greenhouse
against silver leaf on plum, peach and nectarine; Dutch elm disease on elms, honey
fungus (A. mellea) on a range of tree species
and against rots on a wide range of crops,
caused by Fusarium, Rhizoctonia, Pythium
and Sclerotium (Table 10.2). Lacicowa and
Pieta (1994) reported that Trichoderma spp.
and Gliocladium sp. gave significant control
against soilborne pathogenic fungi of pea,
which was better than that obtained with
the use of chemicals. Spiers et al. (2004)
described the mode of action of Trichoderma sp. against plant pathogens. Recently,
Herrera-Estrella and Chet (2004) discussed
the role of Trichoderma spp. as a biological
control agent; the expression of mycoparasitism related genes (MRGs); antibiosis; the
role of MRGs in biological control and strain
improvement; competition; induced resistance; plant growth promotion; and Trichoderma spp. as a source of genes for crop
improvement.
The biocontrol action is due largely to
the inherent nature of inhibition or degradation of pectinases and other enzymes, which
are deemed essential for phytopathogenic
fungi in order to cause pathogenesis in
plants. These direct effects on other fungi
are remarkable yet complex and, until now,
were attributed to being the basis for the
action exerted by Trichoderma sp. on plant
growth and development.
Mechanism of Action of Trichoderma
Several modes of action have been proposed
to explain the suppression of plant pathogens by Trichoderma spp. These include
mycoparasitism, antibiosis, competition,
siderophore production, induction of systemic resistance, growth promotion, etc.
(Dennis and Webster, 1971; Upadhyay and
Mukhopadhyay, 1986; Chet, 1987).
Table 10.2. Trichoderma as biocontrol agents and their target pathogens which cause diseases in
various host plants.
Biocontrol agent
Pathogens
Host crop
Trichoderma spp.
T. harzianum
Pythium sp.
Fusarium oxysporum
Fusarium sp.
Pythium sp.
Rhizoctonia solani
Sclerotinia sclerotiorum
Sclerotium rolfsii
Gaeumannomyces sp.
Pythium sp.
R. solani
Bean, pea, cucumber
Cucumber, cotton, wheat, muskmelon, tomato, ginger
Lentil, cotton
Pea, radish, cucumber, tomato
Pea, radish, snapbean
Cucumber, Mentha sp.
Sugarbeet, groundnut, chickpea, Mentha sp.
Wheat
White mustard
Potato
T. viride
Biological Control of Plant Diseases
Direct action of biocontrol
agent Trichoderma
Mycoparasitism
Mycoparasitism is the phenomenon in which
fungal parasites attack other fungi. It is
divided into necrotrophic (destructive) and
biotropic (balanced) parasitism (Barnett and
Binder, 1973). Trichoderma spp. are grouped
in necrotrophic mycoparsites. Velikanov
et al. (1994) noticed hyperparasitism with
different strains of T. viride, T. harzianum
and G. virens, which were tested against five
phytopathogenic fungi, namely F. oxysporum, F. solani, Pythium sp., R. solani and
S. sclerotiorum causing root rot of pea.
Trichoderma recognizes signals from the
host fungus, triggering coiling and host penetration. Remote sensing is due at least partially to the sequential expression of cell wall
degrading enzymes. Different strains can follow different patterns of induction, but the
fungi apparently always produce low levels
of an extracellular exochitinase. The possible
role of agglutinins in the recognition process
determining fungal specificity has been
examined recently. Barak et al. (1985) proposed that lectins of plant pathogenic fungi
might play a role in recognition. Inbar and
Chet (1992) proved the role of lectins in recognition during mycoparasitism using a
biometric system. Secretion of lytic enzymes,
including b-1,3-glucanase(s), proteinase(s),
chitinases and lipases, enables Trichoderma
spp. to degrade the host cell wall, thereby
reducing the incidence of disease (Harman,
2001). Ordentlinch et al. (1990) reported that
there was no correlation between in vivo and
separated in vitro dual culture or enzyme
assays. Involvement of chitinase and b-1,3glucanase in Trichoderma-mediated biological control was also reported by Harman
(2001). Involvement of b-1,6-glucanases and
b-1,4-glucanases may also play an important
role in mycoparasitism (Thrane et al., 1997).
T. harzianum-mediated mycoparasitism may
involve 20 separate genes and gene products;
most of these gene products are synergistic
with one another (Lorito, 1998).
It is considered that mycoparasitism is
one of the main mechanisms involved in
127
the antagonism of Trichoderma as a biocontrol agent. The process apparently includes:
1. Chemotropic growth of Trichoderma;
2. Recognition of the host by the mycoparasite;
3. Secretion of extracellular enzymes;
4. Penetration of the hyphae; and
5. Lysis of the host.
Antibiosis
The high percentage of effectiveness of the
biocontrol ability of Trichoderma is conferred
most likely by more than one exclusive
mechanism. Another known mechanism of
biocontrol is antibiosis, which is the release
of antibiotics and other metabolites that are
harmful to the pathogen and inhibit their
growth. Many such substances have been
isolated from Trichoderma sp., namely gliotoxin and glyoviridin from T. viride (Sharma
and Dohroo, 1991), viridin, alkyl pyrones,
isonitriles, polyketides, diketopiperazines
and some steroids (Upadhyay and Mukhopadhyay, 1986). Many Trichoderma spp.
are reported to produce volatile and nonvolatile antibiotics, chloroform soluble antibiotics, including trichodermin, and peptide
antibiotics active against a range of plant
pathogenic fungi (Dennis and Webster,
1971). Indeed some isolates of Trichoderma
excrete growth-inhibitory substances. In
fact, it seems advantageous for a biocontrol
agent to suppress a plant pathogen using
multiple mechanisms.
Competition
This mode of action implies the competition among microorganisms for space and
nutrients when these factors are limiting in
nature. It is considered a ‘classical’ mechanism of biocontrol. The mechanism is considered involved when no evidence of either
mycoparasitism or antibiosis is found in a
particular interaction. Since Trichoderma is
an omnipresent fungus and is found in agricultural and natural soils throughout the
world, it is enough proof of it being an
excellent competitor for space and nutritional resources. Excellent competitiveness
for space and nutrition is supposed to be
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A. Tripathi et al.
useful for biological control in the absence
of mycoparasitism or antibiosis (Cook and
Baker, 1983). Elad (2000) reported that when
conidia of T-39 were sprayed on leaves, germination of conidia of B. cinerea was slowed
down, because the pathogenic conidia
required external nutrients for germination
and infection.
Indirect action of biocontrol agents
In addition to the ability of Trichoderma
spp. to attack or inhibit the growth of plant
pathogens directly, recent discoveries indicate that they can also induce systemic and
localized resistance to a variety of plant
pathogens.
Biochemical elicitors of disease
resistance and induced
systemic resistance
Induced systemic resistance (ISR) is another
phenomenon of biocontrol exhibited by the
plant to combat the harmful effects of the
pathogen. It implies the elicitation of resistance or plant response against the microorganism or abiotic agent, such that following
the inducing challenge posed to the plant,
de novo resistance to pathogens is shown in
normally susceptible plants. Localized and
systemic induced resistance occurs in all or
most plants in response to attack by pathogenic microorganisms, physical damage due
to insects or other factors, treatment with
various chemical inducers and the presence
of non-pathogenic rhizobacteria. Specific
strains of fungi in the genus Trichoderma
colonize and penetrate plant root tissues
and initiate a series of morphological and
biochemical changes in the plant, which are
considered to be part of the plant defence
response. Finally, it leads to ISR in the
entire plant. The capability of T. harzianum
to promote increased growth response was
verified both in greenhouse experiments
and in the hydroponic system. A 30% increase
in seedling emergence was observed and
these plants exhibited a 95% increase in
root area. Similarly, an increase in P and Fe
concentration was observed in Trichoderma
inoculated plants.
In recent times, there has been tremendous progress related to pathways of resistance and much has been done to elucidate
them. In many instances, salicylic acid or jasmonic acid, together with ethylene or nitrous
oxide, induce a cascade of events that lead to
the production of a variety of metabolites
and proteins with diverse functions. Different pathways are induced by different challenges, although there seems to be crosstalk
or competition between pathways.
There has been a great leap in explaining the ISR pathway activated by rhizobacteria; the best part is that it is the closest
analogue of induced resistance activated
by Trichoderma. The rhizobacteria-induced
systemic resistance (RISR) pathway phenotypically resembles systemic acquired resistance (SAR) systems in plants. Heil (2001)
defined ISR as the set of changes by which
plants respond to an initial infection or elicitor treatment in becoming systemically
resistant against pathogen attack. Several
workers demonstrated that Trichoderma
spp. could also affect the host plant, which
shows an induced resistance-type response.
Chang et al. (1986) reported hastened
flowering, increased number of blooms in
Chrysanthemum and an increase in the height
and weights of other plants as a result of T.
harzianum inoculation in steamed soil. Trichoderma viride-coated seeds of broad bean
resulted in increased fresh and dry weight of
shoots, roots and nodules (Yehia et al., 1985).
Pea seeds treated with apple pomace-based
Trichoderma inoculant extracts resulted in
increased emergence, rapid plant growth,
increased seedling vigour and phenolics
content. The increase in overall phenolic
content may contribute to improved lignification and antioxidant response (Zheng and
Shetty, 2000). Altomore et al. (1999) reported
for the first time the ability of a Trichoderma
strain (T-22) to solubilize insoluble or sparingly soluble minerals by three possible
mechanisms, namely acidification, production of chelating agents and redox activity.
Further, they reported the solubilization of
Fe2O3, MnO2, Zn and rock phosphate by the
Biological Control of Plant Diseases
cell-free culture filtrate of T-22. Trichoderma strains are also supposed to induce
the production of hormone-like metabolites
on release of nutrients from soil or organic
matter (Kleifeld and Chet, 1992).
Chemicals Produced by Trichoderma
What has been stated above is the induced
resistance exhibited by some plants that is a
result of some microorganism, in this case,
Trichoderma. In this context, it has been
found that Trichoderma produces three
classes of compounds to exert its effect and
induce resistance in plants. These include:
Proteins with enzymatic or
other functions
With regards to the first class of biochemical
elicitors of Trichoderma, it is stated that
much before the discovery of the induction
of resistance by Trichoderma, a small 22-kDa
xylanase protein was shown to induce ethylene production and plant defence. Working
in the direction of Trichoderma, it has been
found very recently that a series of proteins
and peptides that are active in inducing terpenoid phytoalexin biosynthesis and peroxidase activity in cotton are produced by
strains of T. virens.
Avr homologues
Another class is the protein product of Avr
genes, which have been identified in a variety of fungal and bacterial plant pathogens.
These are usually seen functioning as raceor pathovar-specific elicitors, possessing the
capability of inducing hypersensitive responses and other defence-related reactions
in plant cultivars that contain the corresponding resistance gene. Proteome analysis
of T-22 identified proteins that are homologues of Avr4 and Avr9 from Cladosporium
fulvum; T. atroviride strain P1 also produces
similar proteins.
129
Oligosaccharides and low molecular
weight compounds
Another finding in this sphere has been the
transformation of Trichoderma mutants
with reporter based on green fluorescent
protein or specific enzymatic activities (glucose oxidase) under the control of biocontrolrelated promoters. One of the advantages of
this discovery has been the possibility that
biomolecules released by the action of
Trichoderma secreted cell wall degrading
enzymes on the cell walls of fungal pathogens
and plants can be isolated. These molecules
function as inducers of the antagonistic
gene-expression cascade in Trichoderma
and some also function as elicitors of plant
defence mechanisms.
Plant Growth Promotion
Fungal as well as bacterial biocontrol agents
are reported and known to induce growth
of various crops and also increase crop
yield. Trichoderma spp., and other beneficial root-colonizing microorganisms, also
enhance plant growth and productivity.
Mukhopadhyay (1996) has reported increased growth of several crop plants following
seed treatment with T. harzianum and T.
virens. The reason attributed to this effect
of Trichoderma and other microbes on
plants has been explained based on the following arguments.
1. Suppression of harmful root microflora,
including those not a direct causal organism
of disease.
2. Production or activation of growthstimulating factors.
3. Increased nutrient uptake through solubilization and sequestering of nutrients.
It is a well-established fact that microorganisms closely associated with the roots of a
plant can influence plant growth and development directly. Although the ability of
species of Trichoderma spp. to promote or
inhibit plant growth directly has been noted
for many years (Ozbay and Newman, 2004),
efforts to define and exploit these influences
130
A. Tripathi et al.
have met with limited success. Many workers have reported plant growth promotion by
different strains of Trichoderma spp. Chang
et al. (1986) observed plant growth promotion resulting in enhanced germination, more
rapid flowering, increased flowering and
increased height and fresh weight in pepper, periwinkle, Chrysanthemum and several others after treatment of the soil with
peat/bran inoculum or conidial suspension
of T. harzianum.
Solubilization and Sequestration of
Inorganic Plant Nutrients
It is a common natural occurrence that plant
nutrients undergo a complex, intricately
woven conversion from soluble to insoluble
forms when in the soil; this is a precursor to
the ease of access and absorption by roots. It
is here that microorganisms may influence
these transitions (Altomare et al., 1999). The
most commonly and extensively studied
nutrients are iron and manganese. Trichoderma sp. has been reported to produce some
compounds called siderophores (Sen, 2000).
Iron chelated with these siderophores is in
the unavailable and bound form for plant
pathogens and so they do not have access to
iron. On the contrary, plant roots are capable of absorbing iron in this form, so these
are accessible to the plant. This is one of the
mechanisms that operate for the growth of
plants and the supply of nutrients to them.
Trichoderma sp. increases the uptake and
concentration of a variety of nutrients
(copper, phosphorus, iron, manganese and
sodium) in roots of hydroponic culture,
even under axenic conditions. This increased
uptake indicates an improvement in plant
active uptake mechanisms.
Pesticide Susceptibility
Another aspect and quality of Trichoderma
sp. lies in the fact that it possesses innate
and natural resistance against most agricultural chemicals, including fungicides. The
capability differs with strain. Some lines
have been selected or modified to be resistant to specific agricultural chemicals.
Mass Multiplication of Trichoderma
The most critical obstacles to the application
of biological control fungi as an effective
means of disease management are the lack
of knowledge of methods for mass culturing
and a proper delivery system, which is needed
to augment the soil directly with fungal
antagonists (Papavizas, 1985; Singh et al.,
2002, 2004; Dissevelt and Ravensberg, 2004).
Solid media for the experimental production of Trichoderma sp. and Gliocladium
sp., two of the most common fungal antagonists, have been used frequently in laboratory
and greenhouse studies (Bateman, 2004).
Some workers have tried composted
hardwood bark as a substrate for the largescale production of biocontrol fungi (Nelson
and Hoitink, 1983). Sundheim (1977) used
bark pellets as a medium for mass production of Trichoderma and Gliocladium sp. to
control Phomopsis sclerotioides in cucumber. A variety of media have been used by
various researchers for the production of
Trichoderma sp. in stationary flasks, shakers (Jin et al., 1991) and liquid fermenters
(Jin et al., 1996).
Backman and Rodriguez-Kabana (1975)
used diatomaceous earth granules along
with molasses for developing a formulation
of biocontrol agents for application in soil.
Hadar et al. (1979) used wheat bran formulations for mass-multiplying biocontrol agents
for field application. Papavizas et al. (1984)
developed a liquid fermentation technology
for mass production of fungal antagonists by
employing a combination of molasses and
brewer’s yeast. Sivan et al. (1984) developed
a formulation of T. harzianum on wheat bran
and peat. Mukhopadhyay et al. (1986) used
sorghum grains to prepare the powdered formulations of fungal antagonists.
Tapioca rind, cow dung, biogas slurry,
farmyard manure, paddy chaff, rice bran,
groundnut shell, sugarcane bagasse, sheep
manure, chickpea husk, maize cob, etc.,
are some of the substrates used for mass
Biological Control of Plant Diseases
multiplication of T. harzianum and T. viride
(Kousalya and Jeyarajan, 1990). Conway
et al. (1996) used oat seeds for mass culturing of T. harzianum isolate OK-86. Alginate
pellets were used for formulating a biomass
of G. virens and T. hamatum and various
food bases like wheat bran, maize cobs,
groundnut hulls, soy fibre, castor pomace,
cocoa hulls and chitin were used. They
found that the pellets with G. virens and all
the food bases with bran, soy fibres, castor,
pomace or chitin resulted in stands similar
to those of the control, except cocoa hull
meal significantly reduced damping-off of
Zinnia caused by R. solani and P. ultimum.
Kumar and Marimuthu (1997) tested
the effect of decomposed coconut coir pith
(DCCP) added to normal nursery media on
the survival of T. viride. The pure DCCP
gave efficient sporulation of T. viride population. Lewis et al. (1998) used commercially
manufactured cellulose granules (Biodac) in
a mixture with a sticker and fermenterproduced biomass of Trichoderma sp. and
G. virens to produce a formulation in which
chlamydospores in the biomass were activated with dilute acid. Tiwari et al. (2004)
suggested that among the eight substrates,
namely grains of Sorghum vulgare [S. bicolor],
wheat, Pennisetum typhoides [P. glaucum],
S. vulgare cv. M.P. Chari and Sorghum sp.,
a locally available millet; wheat bran; rice
bran; and sugarcane bagasse were evaluated
for the mass propagation of T. viride. Sorghum sp., a locally available millet, resulted
in the greatest spore concentration, spore
viability and total biomass of the fungal
antagonist. The greatest spore concentration
(8 × 109) was observed after 15 days of incubation at 27 ± 1°C. The spores of T. viride
remained viable for 6 months at 5°C.
131
would be beneficial to a larger degree than
individual components.
A primary obstacle in the commercial
use of Trichoderma spp. for both disease control and growth enhancement is the mass
production and delivery methods of its formation to the plants (Papavizas, 1985; Mukhopadhyay, 1996). The problem lies in the
fact that biocontrol products represent living
systems. A large number of growth media are
reported to be suitable for the genus Trichoderma, but most of these are either food
grains or are expensive. For solid-state fermentation substrates like sorghum grain,
wheat grains, wheat bran, tea leaf waste, coffee husk, sawdust, etc., have been used (Gogoi
and Roy, 1996; Mishra, 1998). A liquid fermentation method consisting of molasses,
wheat bran and yeast is proposed for largescale production of Trichoderma (Montealegre et al., 1993). Bioefficacy of T. harzianum
produced by solid fermentation, which contains only conidia, was found more effective
than when produced by liquid fermentation,
where a mixture of chlamydospores, hyphal
fragments and conidia were present.
Conidia of Trichoderma in pyrophyllite survived better than alone at between –5
and 30°C. A temperature range from –5 to
5°C was found most suitable for an improved shelf life (Mukherjee, 1991). Mukherjee
reported that shelf life of T. virens was almost
constant on coated chickpea seeds at 5°C
and, at room temperature, it was decreased
by 12%. Chlamydospore-based formulations
exhibited longer shelf life than conidiabased formulations (Mishra et al., 2001).
Basic Components of
Biocontrol Systems
There are three basic components of biocontrol systems. These are as follows:
Commercial Use of Trichoderma
Commercialized systems for the biological
control of plant diseases are few. It has been
stressed that microbes cannot be used in
isolation and exceptional results expected.
On the contrary, a biocontrol system or
consortia needs to be developed, which
Biocontrol strain
The first step towards successful biocontrol
is to obtain or produce a highly effective biocontrol strain or other material (Table 10.3).
For instance, the development of the T-22
132
A. Tripathi et al.
Table 10.3. Inexpensive production and formulation of the biocontrol agent using various base
materials.
Base material
Biocontrol agent
Formulation
References
Blackgram shell, shelled maize cob,
coir pith, peat, gypsum, barley grains
Coffee fruit skin + biogas slurry
Trichoderma viride,
T. harzianum
T. harzianum
Powder
Coffee husk
T. harzianum,
T. viride, T. virens
T. harzianum,
T. viride, T. virens
T. harzianum,
T. viride, T. virens
T. viride
T. viride
T. harzianum, T. viride
T. harzianum,
T. virens
Kumar and
Marimuthu, 1997
Sawant and
Sawant, 1996
Bhai et al., 1994
Coffee berry husk
Fruit skin and berry mucilage
Groundnut shell
Mustard oil cake
Soil
Sorghum grain
Sugarcane straw
Wheat bran
Rice husk, maize cob powder, spent tea
leaves, wheat bran, citrus fruit pulp
T. harzianum, T. viride,
T. reesei, T. koningii
T. virens
T. harzianum
(MTCC 3843)
strain of T. harzianum by Harman and fellow researchers was the result of a decade
and more of hard work. Still, its commercial
product, Root Shield, picked up pace in the
late 20th century (Harman, 2000). Besides
the usual properties of a biocontrol agent,
the strain must also possess the following: (i)
to be able to compete and persist in the environment in which it must operate and (ii) ideally, to be able to colonize and proliferate on
existing and newly formed plant parts well
after application. Sundaram (1996) developed
fusants of two isolates of T. harzianum (Th-1
and Th-2), among them some showed morphological characters immediately between
Th-1 and Th-3. When T. harzianum (Th-3)
was fused with T. virens, many fusants were
developed and few exhibited improved biocontrol activity (Ghosh, 1996) (Table 10.4).
Ease of delivery and application
Some delivery methods for Trichoderma
are listed in Table 10.5.
Pellets
Pellets
Pellets
Pellets
Powder
Pellets
Powder
Powder
Sawant and
Sawant, 1989
Sawant and
Sawant, 1989
Pellets
Singh, 2002
Upadhyay and Mukhopadhyay, 1986;
Mishra, 1998
Singh et al., 2004
Powder
Powder
Singh et al., 2002
Tripathi, 1998
Compatibility Testing of Trichoderma
The success of a biocontrol agent depends on
its compatibility with other disease management systems. This requires holistic testing
of biocontrol agents (BCA) in combination
with other disease management practices in
a system approach. Once the BCA is found
to be compatible, it can be integrated successfully with the disease management
modules for each cropping system. Csinos
et al. (1983) evaluated the compatibility of
Trichoderma spp. with fungicides for the
management of S. rolfsii in groundnut. T. harzianum, Rhizobium and carbendazim were
integrated successfully for the management
of stem rot of groundnuts caused by S. rolfsii. A combination of either Trichoderma or
Gliocladium with fungicides like carboxin
or metalaxyl protected crop plants against
soilborne pathogens and was emphasized
by several workers (Sawant and Mukhopadhyay, 1990; Mukhopadhyay et al., 1992).
The alternation of BCA with fungicides was
found to be more effective than mixtures.
Biological Control of Plant Diseases
133
Table 10.4. Commercial products of Trichoderma currently in the open market or under registration.
Product
Biocontrol agent
Effective against
Manufacturer/distributor
Antifungus
Trichoderma sp.
Various fungi
Bas-derma
Binab T
Various fungi
Control of wound decay and
wood rot
Bioderma
T. viride Basarass
T. harzianum
(ATCC 20476) and
T. polysporum
(ATCC 20475)
T. harzianum/T. viride
Grondortsmettigen De
Cuester n.v., Belgium
Biocontrol Res. Lab., India
Bio-innovation AB, UK
Biofungus
Trichoderma sp.
Bio-trek 22G
T. harzianum
Sclerotinia, Phytophthora,
Rhizoctonia solani, Pythium
spp., Fusarium, Verticillium
Various fungi
Ecofit
T. viride
Various fungi
Root pro, Root
Protato
T. harzianum
Root shield, Plant
shield, T-22
Planter Box
RUTOPIA
T. harzianum
Rifai strain
KRL-AG(T-22)
Trichoderma sp.
R. solani, Pythium spp.,
Fusarium spp. and
Sclerotium rolfsii
Pythium spp., R. solani,
Fusarium spp.
SoilGard
(formerly
GlioGard)
Supresivit
Trichoderma sp.
T. harzianum
T-22 G,
T-22 HB
Trichoderma
2000
Trichodex,
Trichophel
Trichophel,
Trichoject,
Trichodowels,
Trichoseal
Tri-control
Trieco
T. harzianum strain
KRL-AG2
Trichoderma spp.
Trichoderma sp.
T. viride
TY
Tusal
Trichoderma spp.
Trichoderma spp.
T. harzianum
T. harzianum and
T. viride
Various fungi
Organic Soil Amendment
Turfgrass Biostimulant
Damping-off diseases
caused by Pythium and
Rhizoctonia spp.
Various fungi
Various fungi
R. solani, S. rolfsii,
Pythium spp., Fusarium spp.
Botrytis of vegetables and
grapevines
Armillaria, Botryosphaeria,
Chondrosternum, Fusarium,
Nectria, Phytophthora,
Pythium, Rhizoctonia
Various fungi
Rhizoctonia spp., Pythium spp.,
Fusarium spp., root rot,
seedling rot, collar rot, red rot,
damping-off Fusarium wilt
Various fungi
Damping-off diseases caused
by Pythium, Phoma and
Rhizoctonia species,
rhizomania disease of
sugarbeet and drop of lettuce
Biotech International Ltd.,
India
Grondortsmettigen De
Cuester n.v., Belgium
Bioworks, Inc. of Geneva,
NY
Hoechst Schering Agro
Evo Ltd., India
Efal Agr, Israel
Bioworks Inc., USA
NaEx Corp/Poulenger
USA, Inc
USA
Borregaard and Reitzel,
Czech Republic
THT Inc., USA
Myocontrol Ltd., Israel
Makhteshim Chemical
Works Ltd., USA
Agrimm Technologies Ltd.,
New Zealand
Jeypee Biotechs, India
Ecosense Labs Pvt. Ltd.,
Mumbai, India
Myocontrol, Israel
Spain
134
A. Tripathi et al.
Table 10.5. Mass production and delivery methods of Trichoderma.
Biocontrol agent
Mass production
Trichoderma viride
Commercially produced pellets
Applied directly to the soil along
(BINAB T SEPPIC). Also produced
with food base
on wheat bran: sawdust and tap water
(3:14). Have been produced on a variety
of growth media (autoclaved rye, barley
and sunflower seeds)
As in T. viride; also produced on
Backman and Rodriguez-Kabana
molasses and enriched clay
applied it @ 140 kg/ha after 70
granules as food base
days of planting
T. harzianum
Delivery method
Integration of T. harzianum with a sublethal
dose of methyl bromide (300 kg/ha) and
soil solarization yielded maximum control of
Fusarium crown and root rot of tomato caused
by F. oxysporum f. sp. radicis-lycopersici
(Sivan and Chet, 1993).
In order to get the maximum efficiency
from Trichoderma, it is important that it
should be applied properly. It is effective as
a seed treatment with or without fungicides. The basic reason why this is used is
its multifaceted nature and broad range. It
colonizes roots, increases root mass and
improves plant health, and consequently
provides yield increases, which chemical
fungicides applied at reasonable rates cannot do. It can also be used in conjugation
with other microbes, which thereby increases its efficiency. The two-pronged advantage would be a reduction in the use of
pesticides and limiting root-attacking diseases, plus protection of transplants in the
field by virtue of its ability to colonize
roots. Besides this, powdered formulations
can be made and applied to the seed
directly, and then the seeds are sown. This
would reduce the amount of biocontrol
agent used, as well as protect the plants
from pathogen attack. Further, plant growth
would also improve.
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11
Physiological Specialization of
Ustilaginales (Smut) of Genera Bromus,
Zea and Triticum in Argentina
Marta M. Astiz Gassó1 and María del C. Molina1,2
1Instituto
Fitotécnico de Santa Catalina (IFSC) and 2Consejo Nacional de
Investigaciones Científicas y Tecnológicas (CONICET), Facultad de
Ciencias Agrarias y Forestales, Universidad Nacional de La Plata,
Llavallol, Buenos Aires, Argentina
Abstract
The objective of this project was to determine the existence of physiological forms of Ustilaginales in
Bromus, Zea and Triticum types in Argentina. Studies were carried out on the physiological specialization of Ustilago bullata Berk on Bromus spp., Zea seedlings’ reaction to inoculation with U. maydis
(D.C.) Corda and physiological specialization of Tilletia laevis Wallr. (common bunt) on Triticum spp.
The smut was collected in different agricultural and cattle-raising regions in the country, using Ustilaginales taxonomic keys for smut identification and classification. The experiments were carried out
in greenhouses and in fields at the Instituto Fitotecnico de Santa Catalina (FCAyF-UNLP). For U. bullata and T. laevis, the techniques used were as follows: inoculation by sprinkling of teliospores on host
seeds and inoculation by hypodermic syringe with suspension of U. maydis sporidia on plantlets of
Z. mays and related wild species. As a result of said studies, it was determined that: (i) different
physiological forms exist in each of the kinds of smut analysed; (ii) genetic variability exists in the
hosts which have genes that express different degrees of resistance to the disease; and (iii) genetic
improvement is the most efficient and least environmentally harmful method.
Introduction
Smuts are pathogens of plants that belong to
Phylum Basidiomycota, Class Ustilaginomycetes, Order Ustilaginales. Smut has the
characteristic of forming greyish-black powdery masses of teliospores (basidiospores)
on different organs such as the seeds, stems,
leaves, flowers and fruit of the hosts. Approximately 1400 species of smut are known,
which attack around 75 families of Angiospermae; the most familiar diseases are those
affecting Monocotyledoneae, especially cere138
als, where the pathogens produce important
economic losses (Fischer and Holton, 1957;
Hirschhorn, 1986; Snetselaar and Mims,
1992). Until the 20th century, they were considered, worldwide, one of the most serious
causes of loss of grain and/or seeds, similar
to the effects produced by rust.
In Argentina, between 1934 and 1995,
Hirschhorn and collaborators carried out several studies on Ustilaginales covering the
taxonomic classification of the species, geographical distribution, germination types and
histopathology and cytology of the different
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Physiological Specialization of Ustilaginales (Smut)
species of smut (Hirschhorn, 1986). Currently,
control of these diseases is by means of agrochemicals and, on a smaller scale, by obtaining species with resistant genes through
improvement programmes and studies on
variability of these pathogens.
The objective of this project was to
determine the existence of physiological
forms of Ustilaginales in Bromus, Zea and
Triticum in Argentina:
1. Physiological specialization of U. bullata Berk on Bromus spp.
2. Zea seedlings’ reaction to inoculation
with U. maydis (D.C.) Corda.
3. Physiological specialization of T. laevis
Wall. (common bunt) on Triticum spp.
Physiological Specialization of
Ustilago bullata Berk on Bromus spp.
Head smut (U. bullata Berk) is a pathogen
which affects the growth of various grass
species, especially within the genus Bromus. The disease is initiated when fungal
hyphae penetrate seedlings; the attack develops from the inflorescences at the expense
of the ovaries, forming a typical sorus. Severe
infection affects limbs and glumes, reducing seed and forage production. In the USA,
Fischer and Holton (1957) and Hirschhorn
(1977, 1986) verified experimentally the
existence of genes for resistance, physiological forms and the ability to cross-breed U.
bullata and U. striiformis. Kreizinger et al.
(1947) recorded the different reactions of U.
bullata on Bromus which grew in the mountains and Bromus which grew on the plains;
these experiences indicated that resistant
Bromus varieties and lines could be obtained
by artificial infection under controlled conditions and in the field. Also, 13 physiological forms of the pathogen could be studied
(Meinrs and Fischer, 1953). In New Zealand,
Falloon (1976, 1979a,b) carried out studies
on the effect of U. bullata infection on B.
catharticus. Also, Falloon and Hume (1988)
reported the effects of the pathogen on B.
willdenowii productivity and endurance in
the field. In Argentina, Hirschhorn (1977)
studied teliospore morphological variations
139
in samples of B. catharticus, B. mollis, Hordeum jubatum and H. compresum. The author
also determined that Bromus head smut in
Argentina was represented by U. bullata, U.
bullata cv. macrospora (Hirschhorn, 1977,
1986). Astiz Gassó (1983, 1985, 1994) reported
the presence of genes for resistance to the
pathogen and in vitro U. bullata teliospore
formation, a phenomenon unrecorded for
this pathogen and uncommon in other smuts.
The objective of this work was to determine
the existence of physiological forms in U. bullata populations on several Bromus species.
In this experiment, we used seeds of B.
catharticus. B. parodii, B. brevis, B. auleticus and B. inermis cv. gombaszpuzta were
provided by the Instituto Fitotécnico de
Santa Catalina, FCAyF and Department of
Genetics, and the Experimental Estación
of Pergamino (INTA). The seeds were deinfested with a 2% formaldehyde solution
for 20 min and then washed in sterile water
three times. For identification of the pathogen, spores from each isolate harvested from
plants naturally infected in the field were
examined microscopically (Table 11.1).
Viability of teliospores was tested by plating them in PDA medium 2% (Fischer and
Holton, 1957). The seeds were infested with
teliospores (1.8 × 10 3g teliospore/g seed),
placed in sulphite paper envelopes and
shaken well, so that spores would stick to
the seed. Precautions were taken to avoid
contamination with the different isolates of
the pathogen. Thirty live seeds per isolate
in three replications were inoculated during
4 consecutive years. An uninoculated sample was also included during the study.
Inoculated samples were sown in experimental plots 1.5 m × 0.40 m in three rows
Table 11.1. Ustilago bullata isolates collected in
different localities in Argentina.
Locality
Province
Pergamino
Tres Arroyos
Llavallol
Gowland
General Roca
Check
Buenos Aires
Buenos Aires
Buenos Aires
Buenos Aires
Río Negro
Mixture
140
M.M. Astiz Gassó and M. del C. Molina
with a distance of 0.20 m between them. The
experimental design used was a randomized
complete block. Evaluations in the field
were conducted by head countings, recording the percentage of infection based on the
number of infected and healthy heads. Then,
the average infection for the 4 years was calculated. The level of resistance/susceptibility was determined using a disease rating
scale (Table 11.2).
Isolates showed an 80–90% teliospore
germination, approximately 20–25 h after
they were cultivated on PDA. The teliospore
germination rate increased with temperature from 20 to 25°C, with significant amongpopulation differences. Boguena et al. (2007)
also obtained similar results when they examined the effect of temperature from teliospore germination. Table 11.3 shows the
reaction of the Bromus species tested with
the different U. bullata isolates. Bromus
catharticus was susceptible to all isolates
including the mixture and similar results
were reported for Astiz Gassó and Aulicino
(1999); B. parodii showed similar reactions,
Table 11.2.
bullata.
Disease rating scale for Ustilago
Reaction
Infection (%)
Resistant (R)
Moderately resistant (MR)
Moderately susceptible (MS)
Susceptible (S)
0–5
6–10
11–30
31–100
Table 11.3.
Argentina.
but the levels of infection were lower than
B. catharticus; B. brevis gave a resistant
reaction to isolate Gowland, a moderately
resistant reaction to Pergamino, Llavallol
and the mixture, a moderately susceptible
reaction to isolate Tres Arroyos and a susceptible reaction to General Roca. Similar
results were reported previously by Astiz
Gassó (1983). B. auleticus and B. inermis cv.
gombaszpuzta were resistant to all isolates
and the uninoculated check did not show
any infection.
Four physiological forms in the populations of U. bullata are shown in Fig. 11.1: (i)
Tres Arroyos; (ii) Pergamino and Llavallol;
(iii) Gowland; and (iv) General Roca. The species B. brevis would be the differential host.
Reaction to Inoculation with Ustilago
maydis (D.C.) Corda on Zea seedlings
Ustilago maydis is a smut that promotes the
development of galls in Zea, the relation
with the host being necessary to fulfil its life
cycle. Damage produced in plants by the
presence of corn stunt is: chlorosis, seedling
death and tumours in leaves, stems, ears
and tassels. At first, it was considered that
U. maydis attacked Z. mays and Z. mexicana, but it was later verified that it also
attacked Z. perennis, Z. diploperennis, Z.
parviglumis, Z. luxurians and their hybrids
with the grown species (Hirschhorn, 1986;
Duran, 1987).
Reaction of Bromus species to different Ustilago bullata collected in different localities in
Ustilago bullata isolates
HOSTS
Bromus catharticus
B. parodii
B. brevis
B. auleticus
B. inermis cv.
gombaszpuzta
Nor-inoculated check
Pergamino
Tres Arroyos
Gowland
Llavallol
General Roca Mixture
S
S
MR
R
R
S
S
MS
R
R
S
S
R
R
R
S
S
MR
R
R
S
S
S
R
R
S
S
MR
R
R
0
0
0
0
0
0
Physiological Specialization of Ustilaginales (Smut)
141
100
90
80
Infection (%)
70
60
50
40
Bromus catharticus
Bromus parodii
Bromus brevis
Bromus auleticus
Bromus inermis cv
Non-inoculated check
30
20
10
0
PERGAMINO
TRES
ARROYOS
GOWLAND
LLAVALLOL
GENERAL
ROCA
MIXTURE
Isolates
Fig. 11.1. Reaction of Bromus spp. to U. bullata isolates.
Until 1964, corn stunt did not any have
incidence at the Instituto Fitotécnico de
Santa Catalina, but in that year, a Z. perennis from Jalisco (México) was introduced
and later on Z. mexicana, Z. parviglumis, Z.
luxurians and Z. diploperennis were also
grown and hybridized to Z. mays. As the
hybrids are grown in the field as well as in
the greenhouse, vegetative plants are available throughout the year (Astiz Gassó and
Molina, 1996).
The pathogen multiplies on these plants
with the corresponding increase in the
number of spores disseminated by air and
in the soil. Losses from corn smut range
from 1% to up to 10% of all Zea species and
hybrids are also attacked, depending on the
environmental conditions favouring pathogen development; sweet corn may show
losses approaching 100% from corn smut in
localized areas (Callow and Ling, 1973;
Hirschhorn, 1986; Banuett, 1995; Astiz Gassó
and Molina, 1999).
In this chapter, the results from analysing
the response of Z. mays, Z. perennis and
Z. diploperennis seedlings when they
are inoculated with six populations of
U. maydis are presented. This was done
with the purpose of determining resistance
of the species and/or inbreds to U. maydis.
The host materials used were the population ‘Colorado Klein’, the inbreds SC66,
B73, E624A688 of Z. mays, as well as clones
of Z. perennis and Z. diploperennis. Over a
time period of 2 years, 1296 plants were
inoculated with different strains of U. maydis isolated from the province of Buenos
Aires (Santa Catalina, Balcarce, Necochea
and 25 de Mayo), the province of Entre Ríos
(Paraná) and the province of Córdoba (Río
Cuarto). These strains were cultivated in a
liquid medium of PDB 2% on a shaker for
18–24 h running at 25°C ± 2. The pathogen
was inoculated by puncturing the base of
the seedling with a hypodermic syringe and
the sporidial suspension with concentrations 105–106 sporidia/ml was then forced
up into the leaf whorl (Callow and Ling,
1973; Snetselaar and Mims, 1992, 1993;
Banuett, 1995; Edmunds, 1998; du Toit and
Pataky, 1999). In many previous works, this
method was very successful in producing
disease galls in seedlings (Astiz Gassó and
Molina, 1999).
142
M.M. Astiz Gassó and M. del C. Molina
The trial involved three replications
and a tester (non-treated plants). The plants
were evaluated using a reaction scale to
determine the mean percentage of infection
with U. maydis (Table 11.4). The first symptoms in seedlings were observed 4–6 days
after inoculation and gall development occurred 7–8 days after the treatment (Fig. 11.2).
The behaviour of the host when inoculated with six populations of U. maydis was
analysed in Fig. 11.3. The hosts that reacted
forming galls (grade 4) were cv. Colorado
Klein: Necochea (8.34%) and Balcarce
(2.78%); B73: Río Cuarto (14.15%), 25 de
Mayo (11.11%), Santa Catalina (5.84%) and
Balcarce (1.04%); E642A688: 25 de Mayo
(8.33%) and Santa Catalina (3.34%); SC66:
Río Cuarto (4.55%); Z. perennis: Santa Catalina (1.67%) and Z. diploperennis: 25 de
Mayo (13.89%), Paraná (2.78) and Santa
Catalina (1.67%).
Table 11.4.
Reaction scale in hosts.
Behaviour
Host reaction
0 = Immune
1 = Resistant
2 = Medium
resistant
No reaction
Partial chlorosis
Accent chlorosis and/or
presence of stripe or
anthocyanin stain
Necrosis and reduction
of growth in plant
Formation of tumours
3 = Medium
susceptibility
4 = Susceptibility
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 11.2. Reaction of hosts after inoculations with U. maydis. (a) No reaction, immune; (b) partial
chlorosis; (c–d) accent chlorosis and/or presence of stripe or anthocyanin stain; (e) necrosis and
reduction of growth in plant; (f) formation of tumours (galls).
Physiological Specialization of Ustilaginales (Smut)
143
% reaction
% reaction
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Pobl.
Pobl. Pobl.25 Pobl. Pobl.Rio Pobl.Sta.
Necochea Balcarce de Mayo Parana Cuarto Catalina
(b)
Isolates
Isolates
50.00
% reaction
60.00
% reaction
20.00
0.00
Pobl.
Pobl. Pobl.25 Pobl. Pobl.Rio Pobl.Sta.
Necochea Balcarce de Mayo Parana Cuarto Catalina
(a)
40.00
40.00
20.00
0.00
30.00
20.00
10.00
0.00
Pobl.
Pobl. Pobl.25 Pobl. Pobl.Rio Pobl.Sta.
Necochea Balcarce de Mayo Parana Cuarto Catalina
(c)
40.00
Pobl.
Pobl. Pobl.25 Pobl. Pobl.Rio Pobl.Sta.
Necochea Balcarce de Mayo Parana Cuarto Catalina
(d)
Isolates
Isolates
100.00
80.00
% reaction
% reaction
80.00
60.00
40.00
20.00
40.00
20.00
0.00
0.00
Pobl.
Pobl. Pobl.25 Pobl. Pobl.Rio Pobl.Sta.
Necochea Balcarce de Mayo Parana Cuarto Catalina
(e)
60.00
(f)
Isolates
Grade 0
Pobl.
Pobl. Pobl.25 Pobl. Pobl.Rio Pobl.Sta.
Necochea Balcarce de Mayo Parana Cuarto Catalina
Grade 1
Isolates
Grade 2
Grade 3
Grade 4
Fig. 11.3. Reaction of Zea mays (lines and populations), Zea perennis and Zea diploperennis to six
strains of U. maydis isolates: (a) Colorado Klein (Z. mays); (b) Lines E642A688 (Z. mays); (c) Line SC66
(Z. mays); (d) Line B73; (e) Z. perennis; and (f) Z. diploperennis.
Physiological Specialization of
Tilletia laevis Wallr. (Common Bunt)
on Triticum spp. in Argentina
Common bunt of wheat is caused by T. tritici and T. laevis; infection takes place in the
coleoptile when teliospores are found on
the coleoptile surface and/or the ground
(Fischer and Holton, 1957; Hirschhorn, 1986;
Wilcoxson and Saari, 1996). Chemical control is achieved through seed treatments;
however, the disease is aggravated due to
inefficiency in the method of fungicide
application and the widespread use of susceptible wheat cultivars. In common bunt,
the spores survive in the soil for long periods and can cause infection of seedlings.
The most effective control method is by
genetic resistance to the pathogen and by
establishing the variability or physiological
specialization of Tilletia species. Historically, pathogenic races that are virulent to
resistant cultivars have appeared, so new
germplasm is screened continually for resistance. Investigations to determine disease
resistance were incorporated into breeding
programmes (Meinrs and Fischer, 1953;
Kendirck, 1961; Metzger and Hoffmann,
1978; Gaudet, 1990; Johnsson, 1991; Gaudet
et al., 1994; Wilcoxson and Saari, 1996).
In Argentina, Hirschhorn and collaborators studied the morphology, taxonomy,
symptomatology, spore germination, basidial
cytology and geographical distribution of
pathogens, T. tritici and T. laevis, to common
bunt of wheat (Hirschhorn, 1986; Astiz Gassó,
1992; Astiz Gassó and Hirschhorn, 1994).
The presence of 12 T. foetida (= T. laevis)
144
M.M. Astiz Gassó and M. del C. Molina
physiological forms and cultivar wheat differentials for identification of T. foetida were
reported by Astiz Gassó (1992, 1997a,b) and
Astiz Gassó and Hirschhorn (1994). The
objective of this work was to establish the
physiological forms of T. laevis and to study
the reaction of commercial wheat cultivars
to the pathogen in Argentina.
In this experiment, we used ten hexaploid bread wheat cultivars with different
levels of resistance and two tetraploid cultivars considered resistant. Seeds were deinfested with a formaldehyde solution (3:1)
and washed in sterile water. Pathogens from
25 localities in the Argentine wheat belt
were tried. Wheat cultivars were inoculated
with 0.5 g of teliospore/100 g of seed. Experimental field plots consisted of three rows
2 m long per cultivar/pathogen isolate. Field
evaluations were carried out by head countings and the percentage of infection was
Table 11.5.
based on the number of infected and healthy
heads. Results were transformed through
the Arcosen and the average for 6 years of
testing was calculated. Data were subjected
to ANOVA (Statistix, 2008). Where significant differences were detected, treatment
means were separated using HSD Tukey
test (P < 0.05). Our field research to date
indicates that T. laevis shows several physiological forms: Tandil, Rio Cuarto, Villa
María, Cabildo, Castelar and Casilda. The
rest of the 19 populations of common bunt
showed homogeneous behaviour, so it
could be considered as one physiological
form (Table 11.5).
Tetraploid cultivar, Buck Cristal, proved
the presence of resistant genes. The hexaploid wheat cultivars, Buck Ñapuca and
Buck Yapeyú, were moderately resistant to
pathogen incompatibility to different isolates (Table 11.6). The rest of the hexaploid
Means of infections of 25 T. laevis populations.
Tilletia laevis populations
Province
Mean
1. Tandil
2. Río Cuarto
3. Bordenave Col.1
4. Sta Rosa
5. Venado Tuerto
6. Tres Arroyos Col.1
7. Lincoln
8. Tres Arroyos Col.2
9. Laboulaye
10. Rafaela
11. Pergamino
12. San Francisco
13. Villa María
14. Salliquelo
15. Marcos Juarez
16. Necochea
17. Cabildo
18. Bordenave Col.2
19. Cañada de Gomez
20. Bragado
21. Río Tercero
22. Paraná
23. Castelar
24. Casilda
25. Bolivar
Buenos Aires
Córdoba
Buenos Aires
La Pampa
Santa Fé
Buenos Aires
Buenos Aires
Buenos Aires
Córdoba
Santa Fé
Buenos Aires
Córdoba
Córdoba
Buenos Aires
Córdoba
Buenos Aires
Buenos Aires
Buenos Aires
Santa Fé
Buenos Aires
Córdoba
Entre Ríos
Buenos Aires
Santa Fé
Buenos Aires
21.50 a
19.83 ab
17.12 abc
16.60 abcd
16.45 abcd
15.70 abcde
15.58 abcde
15.38 abcde
14.48 abcde
14.10 abcde
14.01 abcde
13.70 abcde
12.92 bcde
12.61 bcde
11.89 bcde
11.87 bcde
10.91 cde
10.73 cde
10.30 cde
19.49 cde
19.07 cde
18.32 cde
18.28 de
17.29 e
17.17 e
Note: Means followed by different letters within column indicate significant
differences according to Tukey’s test (P < 0.05).
Physiological Specialization of Ustilaginales (Smut)
Table 11.6. Means of infection of common bunt
in wheat cultivars.
Hosts
Mean
Buck Charrua
Buck Ombú
Buck Catriel
Buck Bagual
Buck Fogón
Buck Guaraní
Buck Ñapuca
Buck Yapeyu
Buck Cristal
19.97 a
18.78 a
17.07 ab
16.58 ab
13.01 bcd
11.21 bcd
18.88 cd
18.72 e
12.85 f
Note: Means followed by the same letter with a column
indicate cultivars that are homogenous according to
Tukey’s test (P < 0.05).
wheat was moderately susceptible. Also, the
interaction among wheat cultivar populations
of T. laevis was significantly high and the
interaction among pathogen population
replications was significantly high according to Tukey’s test (P < 0.05).
Conclusions
From this analysis, four physiological forms
of U. bullata were found in the isolates
studied: Tres Arroyos, Pergamino and
145
L1avallol, Gowland and General Roca. Bromus brevis is the differential host for the
fungus populations and shows genetic resistance to the Gowland isolate. Bromus auleticus and B. inermis cv. gombaszpuzta were
resistant to all the fungus isolates. This was
the first report in Argentina determining the
physiological forms of smut U. bullata of
Bromus spp. It can be concluded that the
wild species and the grown species of the
genus Zea reacted in different ways (tolerant and/or resistant to moderately susceptible), depending on the geographic origin of
U. maydis populations. These results might
be considered when selecting germplasm to
obtain new forage plants from interespecific
hybrids of the genus Zea. The wheat cultivars evaluated would also be used as differentials for identification of T. laevis races.
Six physiological forms were detected
among the used populations of T. laevis.
This is the first report in Argentina determining the physiological forms of smut T.
laevis of Triticum spp. The most effective
methods to control the disease are genetic
resistance and establishing the variability of
the smut populations. Determination of the
physiological forms of U. bullata, U. maydis
and T. laevis and genetic improvement is
the most efficient and least environmentally
harmful method.
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Hirschhorn, E. (1986) Las Ustilaginales de la Flora Argentina. Edit Comisión de Investigaciones Científica
de la provincia de Buenos Aires. Publicación Especial. CIC, 530 pp.
Johnsson, L. (1991) Climate factors influencing attack of common bunt (Tilletia caries (D.C.) Tul) in winter
wheat in 1940–1988 in Sweden. Journal of Plant Diseases and Protection 99(1), 21–28.
Kendirck, E.L. (1961) Race groups of Tilletia caries and Tilletia foetida for varietal resistance testing. Phytopathology 51, 537–540.
Kreizinger, E.J., Fischer, G.W. and Law, A.G. (1947) Reactions of mountain brome and Canada wild-rye
strains to head smut (Ustilago bullata). Journal of Agricultural Research 75, 105–111.
Meinrs, J.P. and Fischer, G.W. (1953) Further studies of host specialization in the head smut of grasses,
Ustilago bullata. Phytopathogy 43, 200–203.
Metzger, R.J. and Hoffmann, J.A. (1978) New races of common bunt useful to determine resistance of
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Snetselaar, K.M. and Mims, C.W. (1992) Sporidial fusion and infection of maize seedlings by the smut
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Snetselaar, K.M. and Mims, C.W. (1993) Infection of maize stigmas by Ustilago maydis: light and electron
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Disease Management. CIMMYT, México, 66 pp.
Part IV
Endophytes in Plant Disease Control
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12
Status and Progress of
Research in Endophytes from
Agricultural Crops in Argentina
Silvina Larrán and Cecilia Mónaco
Centro de Investigaciones de Fitopatología (CIDEFI), Facultad de Ciencias
Agrarias y Forestales, Universidad Nacional de La Plata, La Plata, Argentina
Abstract
Plants harbour a heterogeneous population of endogenous microorganisms, comprising both pathogens and non-pathogens, including fungi, bacteria, actinomycetes, etc. Their association has substantial impact on plant health and fitness. Endophytes reside inside healthy plant tissues without
producing any disease symptoms. They are helpful in modifying biochemicals produced by plants and
may add to their protection from insect herbivores, fungal pathogens and even grazing by animals.
However, the ecological role of these endophytes is not yet fully understood. This chapter reports on
endophytic fungi present in beet and tomato leaves. Isolation and analysis of endophytic microorganisms of soybean and wheat are also described. It is advocated that endophytes may have a definite role
in the biological control of Drechslera tritici-repentis, responsible for tan spot disease in wheat.
Introduction
Before beginning, the term ‘endophyte’
must be defined. Literally, an endophyte is
an organism which lives inside a plant,
‘endo’ meaning within and ‘phyte’ is derived
from the Greek word ‘phyton’, meaning plant.
There are several definitions of endophytes,
such as ‘endophyte’ is an all-encompassing
topographical term that includes all organisms that are living in plant tissues during a
more or less long period of their life, colonizing symptomlessly the living internal
tissues of their hosts (Petrini, 1991). Such
infections are termed ‘endophytic’, particularly when the association is believed to be
mutualistic or at least non-pathogenic, or
‘latent infections’, where a latent pathogen
is involved (Cabral et al., 1993). Therefore,
the term endophyte has been used lately in
a broad sense to include any fungi isolated
from symptomless plant tissues, but the concepts of endophytic colonization and latent
infection by fungi are clearly different. Endophytic colonization or infection cannot be
considered as causing disease, since a plant
disease is an interaction between the host,
parasite, vector and the environment over
time, which results in the production of disease signs and/or symptoms. Endophytic
fungi may be described as mutualistic (Clay,
1991). Latent infecting fungi are parasitic
but cannot be considered mutualistic. Latent
infection is the state in which a host is infected
with a pathogen, but does not show symptoms and persists until signs or symptoms
are prompted to appear by environmental or
nutritional conditions or by the state of
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
149
150
S. Larrán and C. Mónaco
maturity of the host or pathogen (Sinclair
and Cerkauskas, 1996).
Petrini’s definition of endophytes (1991)
encompasses not only mutualistic and neutral symbionts, but also those pathogens
known to live latently within their hosts.
Therefore, Wilson (1995) has expanded the
‘endophyte’ definition to include internal
bacteria that live inside plant tissues without causing disease. A wide range of bacterial genera has been isolated from healthy
plant species of agricultural and horticultural crops (Chanway, 1996, 1998; Sturz
et al., 1998). Endophyte associations may
range from intimate contact where the fungus inhabits the intercellular spaces and
xylem vessels in the plant, to more or less
superficial colonization of peripheral, often
dying or dead tissues (Petrini, 1996). They
may colonize single cells (Stone et al., 1994)
or tissues (Schulz et al., 1999).
The endophytes of aerial parts of plants
could be assembled in two different groups:
the fungal endophytes of grasses and nongrass endophytes (fungi and bacteria). Grass
endophytes are a particular type of systemic
symbiosis, these are fungi of the family Clavicipitaceae, which grow between host cells in
vegetative tissues, ovules and seeds and are
seed transmitted vertically (Stone and
Petrini, 1997). Most studies of endophytes
have dealt with grasses due to their economic
importance to livestock (Clay, 1988, 1991).
The close association of an endophyte (Neotyphodium coenophialum (Morgan-Jones &
Gams) Glenn, Bacon & Hanlin = Acremonium coenophialum) and tall fescue (Festuca
arundinacea L.) has been widely studied. As
a result of the association between host and
fungus, alkaloids are produced. These are
responsible for fescue toxicosis in livestock
(Bacon et al., 1977).
Since the initial work of Bacon et al.
(1977), numerous researchers have come to
understand further the relationship between
fungal endophytes of grasses and animal
toxicosis. On the other hand, it has been
well documented that grass endophytes
provide their host with a number of benefits
that increase host fitness. The most intensely
studied symbioses are tall fescue with N.
coenophialum and perennial ryegrass with
N. lolii (Latch, Samuels & Christensen) Glenn,
Bacon & Hanlin. In tall fescue, N. coenophialum causes enhanced tillering and root
growth, increases drought tolerance (Arechavaleta et al., 1989) and protects against certain nematodes (Kimmons et al., 1990),
fungal pathogens (Gwinn and Gavin, 1992)
and insect herbivores (Rowan and Latch,
1994). The protective nature of endophytes
is due to the presence of alkaloids, whereas
these alkaloids are responsible for poisoning
domestic animals. Ergovaline is associated
with various maladies often observed among
cattle that graze N. coenophialum-infected
tall fescue and collectively called ‘tall fescue toxicosis’. Likewise, lolitrem B is associated with the malady ‘ryegrass staggers’,
most commonly observed in sheep grazing
perennial rye grass in New Zealand (Schardl
and Phillips, 1997).
On the other hand, symptomless endophytes of plants other than grasses have
been known for more than 80 years (Lewis,
1924; Carroll and Carroll, 1978; Fisher et al.,
1992; Menendez et al., 1995; Faeth and
Hammon, 1997; Gasoni and Stegman, 1997;
Fröhlich et al., 2000; Larran et al., 2007).
Endophytes are found in all plants and are
extremely abundant and very diverse. Endophytes of a non-grass host represent a broad
range of genera. Taxonomically, the endophytic fungi recovered from plants belong
mainly to the phylum Ascomycota and
Basidiomycota (fungi) and some Oomycetes
(phylum Oomycota, Chromista) have been
isolated as endophytes (Sinclair and Cerkauskas, 1996), along with members of phylum
Ascomycota and their conidial form or anamorphic form lacking a sexual state.
The strategy of endophytes is commonly
characterized by early occupation of living
host tissue, ensuring possession of the nutritional resource (Dingle and McGee, 2003).
Host colonization by these fungi is frequently
localized in foliage, roots, stems and bark
and they are transmitted horizontally via
spores. Frequently, colonization is more
often non-systemic. These endophytic infections are often presumed to form mutualistic
association with their hosts in a manner similar to the endophytes in grasses (Stone and
Petrini, 1997). The plant tissues act as host
Research in Endophytes from Agricultural Crops in Argentina
for complex fungal communities. In the past
few years, several works have provided evidence for the development of a highly specific endophytic assemblage for a given host
(Bertoni and Cabral, 1988; Petrini and Fisher,
1988; Sieber et al., 1988, 1991; McInroy and
Kloepper, 1991; Pereira et al., 1999; Larran
et al., 2000, 2001, 2002a,b). Organ specificity, probably the result of adaptation by some
endophytes to the particular microecological
and physiological conditions present in a
given organ, has been demonstrated in several studies (Fisher et al., 1991; Petrini et al.,
1992). Whereas a large number of species
can be isolated from a given host, in general,
only a few species are present in significant
amounts (Petrini et al., 1992). The ecological
roles of endophytes are not yet clarified in all
associations. Only the interaction of Neotyphodium/grass has been studied in depth,
but less is known about other endophytic
associations (Clay, 1990).
The endophytes may provide a rapidly
evolving defence mechanism against herbivory (Carroll, 1988, 1991; Findlay et al.,
1995) and many are potential producers of
secondary metabolites and enzymes that
will probably find diverse applications in
the most diverse fields of biology (Petrini
et al., 1992; Schulz et al., 1995; Istifadah and
McGee, 2006; Istifadah et al., 2006). Several
studies have demonstrated auxin and cytokinin production (Pugh, 1972; Bacon and
De Battista, 1991) and antibiotic compounds
(Clark et al., 1989; Brunner and Petrini, 1992).
Competition for infection site, their capacity to produce secondary metabolites and
their potential to stimulate defence reactions may contribute to antagonism by the
endophytes against pathogens living in the
same tissues (Dingle and McGee, 2003; Istifadah and McGee, 2006).
Also, several authors have proposed that
endophytes could be used as vectors of genes
to be introduced artificially in the population of the host, due to natural genomes
showing useful characteristics and attributes
that could be selected. For example, endophytes used as vectors of genetic information
could also be of particular interest for the
development of mycoherbicides (Petrini
et al., 1992). The knowledge of endophyte
151
distribution, biodiversity and biochemical
characteristics could be important in improving plant fitness. Moreover, they could play
an important role in the interactions present
in an ecological agriculture.
In the past few years, research on endophytes has been carried out at the CIDEFI
Research Centre in the city of La Plata, Buenos Aires, Argentina. It is thought that endophytes could be used as biocontrol agents. In
Argentina nowadays, biological control is
an attractive option for the management of
some plant diseases. A considerable amount
of knowledge on endophytes has been accumulated. Preliminary studies have focused
mainly on determining the biodiversity of
endophytes on economically important
plants. Likewise, species composition from
different organs has been investigated.
Finally, research will be undertaken to test
the antagonistic interactions between endophytes and plant pathogens. Significant
research is summarized in this chapter.
Endophytic Fungi in Beet
(Beta vulgaris var.
esculenta L.) Leaves
The aim of this investigation was in order to
document the species composition of endophytic fungi of healthy cultivated beet leaves;
to determine their infection frequencies and
to verify possible qualitative and quantitative changes of species isolated during the
growing season (Larran et al., 2000). Samples were collected from healthy beet leaves
of plants cultivated in the experimental
field of the Facultad de Ciencias Agrarias y
Forestales, Universidad Nacional de La
Plata (UNLP), Buenos Aires, Argentina. The
plants were sampled three times during the
growing season. Leaves were cut, surfacesterilized and then leaf disks were incubated
on 2% potato dextrose agar (PDA) for 8 days.
Nested ANOVA and Tukey tests were applied
to evaluate the differences in infection frequencies for different fungi. Data were transformed according to y = arcsin R2 (P/100).
Microscopic examinations were made from
leaf disks previously surface-sterilized and
152
S. Larrán and C. Mónaco
then incubated in a humid chamber for 48 h.
The disks were cleared and stained. Hyphae
were the principal fungal structures observed
(Fig. 12.1). They could be observed emerging through the stomata or growing intercellularly under the cuticle and could be
followed between the layers of cells. No visible disruption or impairment of the plant
cells by the fungi was noted. The endophytes isolated from healthy beet leaves are
shown in Table 12.1.
Fungi colonized 100% of the leaves
sampled. Twelve taxa of endophytic fungi
were isolated and identified. Yeast, Alternaria alternata, Pleospora herbarum, Stemphylium sp. and Epicoccum nigrum were
the most frequently isolated fungi. The frequency of A. alternata and P. herbarum
increased significantly in time, whereas
yeast decreased along the growth stages.
There were no relevant quantitative changes
in the frequency of colonization by other
species. The diversity of isolated fungi
species decreased from the first to the last
sampling.
Fig. 12.1.
Hyphae emerging from stomata.
Endophytic Fungi in the Leaves of
Lycopersicon esculentum Mill.
We have selected tomato plants for this
investigation because both greenhouse and
field production in La Plata horticultural
area are economically important (Larran
et al., 2001).
Tomato production is used mainly for
fresh consumption, as well as being a source
of many value-added products. The investigation reports the endophyte frequencies
from healthy tomato leaves (cultivar Tommy)
cultivated in the field of the Facultad de
Ciencias Agrarias y Forestales, UNLP, Buenos Aires, Argentina. Samples were collected
for 2 years to determine possible qualitative
and quantitative changes of species. Data
were analysed by ANOVA for factorial experiments. Differences between means were
separated with Tukey’s test (P ≤ 0.05). Likewise, different surface-sterilized techniques
were evaluated previously and the technique
selected was used. The diversity of isolated
endophytes is shown in Table 12.2.
Research in Endophytes from Agricultural Crops in Argentina
153
Table 12.1. Mean density of colonization (%) of endophytic fungi from beet leaves at three different time
intervals during the growing season.
Sampling dates
Endophytes
1
12.5a**
1.0
1.0
1.0
5.3
2.0
1.0
0
1.0
7.3
6.1
18.0
0
11
Alternaria alternata (Fr.) Keissler
Chaetomium sp.
Cladosporium spp.
Colletotrichum dematium (Pers.) Grove
Epicoccum nigrum Link.
Glomerella cingulata (Stonem.) Spaulding & Schrenk
Penicillium spp.
Phoma betae Fr.
Phomopsis sp.
Pleospora herbarum (Pers. ex Fr.) Rabenh.
Stemphylium sp.
Yeast
Sterile mycelia
Total number of endophytes
Total segments sampled: 300
2
3
23.0**
0
1.0
0
3.0
1.0
4.0
1.0
0
9.0
9.0
10.0
1
10
31.0**
0
1.0
0
3.0
0
1.0
0
0
11.0
8.0
7.0
0
7
Note: aMean of ten replications. Numbers followed by ** differ statistically according Tukey’s test (P ≤ 0.05).
Table 12.2. Mean frequencies (%) of endophytic fungi isolated from tomato
leaves in 1998 and 1999.
Mean frequencies (%)
Endophytes
1998
1999
Alternaria alternata (Fr.) Keissler
Arthrinium sp.
Bipolaris cynodontis (Marig.) Shoem.
Chaetomiun globosum Kunze ex Fries
Cladosporium sp.
Colletotrichum coccodes (Wallr.) Hughes
C. gloeosporioides (Penz.) Sacc.
Epicoccum nigrum Link.
Cryptococcus sp.
Nigrospora sphaerica (Sacc.) Mason
Penicillium spp.
Phomopsis sp.
Ulocladium alternariae (Cooke) Simmons
Stemphylium botryosum (Pers.ex Fr.) Rabenh.
Rhodotorula sp.
8.75
3.78
0
2.50
3.75
2.50
13.75*
0
0
2.50
2.50
3.75
2.50
1.25
0
25.8*
0
1.44*
0
5.48
0
0
1.59*
1.87*
0
2.55
0
0
0
2.25*
Note: Means followed by * differ significantly according to Turkey’s test (P ≤ 0.05).
Total segments sampled at each growth stage: 75.
Different endophytic species were isolated in 1998 and 1999, although some of
them were isolated in both years. This could
be due to the different climatic conditions
registered, as several authors observed that
various climatic conditions – site moisture,
rainfall and wind exposure – yielded
distinct endophyte assemblages (Chapela,
154
S. Larrán and C. Mónaco
1989; Petrini et al., 1992). Alternaria alternata was the fungus isolated most frequently
from tomato leaves in 1999, but it was the
second most common species in 1998. In
contrast, C. gloeosporioides was the fungus
isolated most frequently in 1998, but it was
not found in 1999. Species of other genera,
such as Cladosporium and Penicillium, were
isolated in both years. These two genera have
been described as endophytes from other
plants as well (Fisher et al., 1992; Cabral
et al., 1993).
Endophytic Fungi in
Healthy Soybean Leaves
Soybean (Glycine max (L.) Merr.) in Argentina is one of the most important crops, not
only by its production but also because of the
volume exported, and it is planted on about
16.5 m ha. A study (Larran et al., 2002b) was
undertaken to document the diversity of
endophytic fungi of healthy cultivated soy-
bean leaves and their infection frequency
and to verify possible qualitative and quantitative changes of species isolated at two
growth stages: R2–R3 and R4–R5 (according to
Fehr et al., 1971). Fifty asymptomatic plants
were randomly sampled at each growth stage
from a segregating population (F3 generation)
cultivated at the experimental field of the
Facultad de Ciencias Agrarias y Forestales,
UNLP, Buenos Aires, Argentina. Samples
were surface-sterilized and incubated over 9
days. The student t-test and percentage differences test were used to evaluate differences in infection frequencies for various
fungi. The results are shown in Table 12.3.
Twelve genera of endophytic fungi were isolated and identified from healthy soybean
leaves. In general, in both growth stages, the
same species were isolated and most of them
did not show significant differences in their
infection frequencies, except for Phomopsis
sp., P. longicolla and Cladosporium sp.
The endophytic fungi isolated more frequently from healthy leaves of soybean were
Table 12.3. Mean percentage frequencies of endophytic fungi and their variations from soybean leaves
at R2–R3 and R4–R5 stages (total segments sampled: 591).
Frequencies (%)
Endophytes
Alternaria alternata (Fr.) Keissler
A. tenuissima (Kunze ex Pers.) Wiltshire
Bipolaris sorokiniana (Sacc.) Shoem.
Cladosporium sp.
Colletotrichum sp.
Curvularia lunata (Wakker) Boedijni
Epicoccum nigrum Link.
Glomerella cingulata (Stoneman)
Spauld. & Schrenk
G. glycines Lehm. & Wolf
Nigrospora sphaerica (Sacc.) Mason
Penicillium sp.
Phomopsis longicolla Hobbs
P. sojae Lehman
Phomopsis sp.
Pleospora herbarum (Pers. ex Fr.) Rabenh.
Stemphylium sp.
R2–R3 stagea
R4–R5 stage
Variation (%)
78.48b
0
0.94
0
1.28
0
1.23
17.20
68.79
1.60
0
2.06
0
0.40
1.93
14.04
–12.34
–
–100.00
–
–100.00
–
+56.90
–18.40
NS
NS
NS
*
NS
NS
NS
NS
0.51
1.33
0
1.99
3.48
2.89
0.66
3.32
0.40
2.00
1.86
0
3.86
5.50
0.99
2.99
–21.60
+50.40
–
–100.00
+10.90
+90.30
+50.00
–9.90
NS
NS
NS
*
NS
**
NS
NS
Note: aBased on the plant stages designated by Fehr et al. (1971). bThe infection frequency was calculated as the
number of subsamples infected by a given fungus divided by the total number of subsamples incubated. *Significant
difference (P < 0.05); **highly significant difference (P < 0.01); NS, no significant difference.
Research in Endophytes from Agricultural Crops in Argentina
A. alternata and G. cingulata. Most of the
fungi isolated in this work are cited as soybean pathogens in different parts of the world
(Farr et al., 1989). Because it is known that
most fungal pathogens of soybean have an
asymptomatic or latent period after infection
or colonization, these fungi could be either
avirulent or hypovirulent, or virulent but in
a latent phase. Pathogenicity tests would be
needed to investigate this hypothesis. Soybean leaves are hosts to an abundance of
endophytic fungi, but only A. alternata is the
dominant species. Further studies will be
carried out to evaluate the potential use of
endophytes from soybean leaves in biological control.
Isolation and Analysis of Endophytic
Microorganisms in Wheat Leaves
The presence of endophytic fungi in healthy
wheat crops has been demonstrated previously in other countries of the world. The
present investigation was undertaken in order
to document the spectrum of endophytes of
healthy leaves from three wheat cultivars
and to determine their infection frequencies
at three growth stages in Argentina (Larran
et al., 2002a). Wheat cultivars, Buck Ombú,
Klein Centauro and Klein Dragón, were
grown in the experimental field of the Facultad de Ciencias Agrarias y Forestales, UNLP,
Buenos Aires, Argentina. Ten asymptomatic
plants of each cultivar were randomly sampled at three defined growth stages: second
node detectable, medium milk and soft dough
stages (32, 75 and 85, according to Zadoks
et al., 1974). Samples were surface-sterilized
and incubated on 2% PDA and, after 9 days,
identifications were made. Data were analysed by ANOVA for factorial experiments.
Differences between means were separated
by LSD (P ≤ 0.05).
From the 450 wheat leaf segments incubated, 3 bacterial isolates and 130 fungal
isolates were obtained (Table 12.4). From
all the isolates, 19 fungal species were identified. There were significant differences
between microorganisms, stages of growth
and stages × microorganism interactions. Differences between cultivars, stages × cultivars,
155
microorganisms × cultivars and the triple
interaction were not significant. The frequency of the microorganisms isolated
increased with crop age, but it was statistically similar for the three wheat cultivars
tested. Rhodotorula rubra, A. alternata, C.
herbarum and E. nigrum were isolated in
the highest frequency. The other microorganisms were present at intermediate or low
values. Most fungal endophyte isolates from
wheat leaves have been described as endophytes of wheat and others plants (Sieber
et al., 1988; Petrini et al., 1992; Gindrat and
Pezet, 1994).
A variation in the number of taxa isolated was recorded along the growing season
of wheat. A change in species composition
from the three growth stages was observed;
however, no differences were noted between
cultivars. Further studies were needed to
analyse endophyte composition and variation from other organs and cultivars. Therefore, the following study was undertaken.
Endophytic Fungi from Wheat
(Triticum aestivum L.)
In this work, five wheat cultivars (Buck Poncho, B. pronto, Klein Cobre, K. Dragón and
Pro INTA Federal) were grown in the experimental field of the Facultad de Ciencias
Agrarias y Forestales, UNLP, Buenos Aires,
Argentina. The purpose of this investigation
was to document the diversity of endophytes
from different cultivars and to determine
their infection frequencies from different
plant organ (leaves, stems, glumes and grains)
(Larran et al., 2007). Samples were collected
at five growth stages from crop emergence
to harvest (GS 2, GS 8, GS 10.5, GS 11.1 and
GS 11.4) (Large, 1954), with the aim of verifying possible qualitative and quantitative
changes of the species isolated. Pieces of
tissues were surface-sterilized and incubated on 2% PDA over 9 days. An ANOVA
including organs, microorganisms, cultivars
and growth stages as a source of variation was
carried out but, due to differences between
organs, an ANOVA was performed considering each organ separately. Differences
between means were separated by LSD
156
S. Larrán and C. Mónaco
Table 12.4. Frequencies of endophytes isolated from wheat leaves of three cultivars at three
growth stages.
Samplings
Endophytes
Gs. 35*
Gs. 75
Gs. 85
Average
Alternaria alternata (Fr.) Keissler
Alternaria sp. I
Alternaria sp. II
Arthrinium sp.
Aspergillus sp.
Bipolaris sp.
B. cynodontis (Marig.) Shoem.
B. sorokiniana (Sacc.) Shoem.
Chaetomium globosum Kunze ex Fries
Cladosporium herbarum (Pers.: Fr.) Link.
Cryptococcus sp.
Epicoccum nigrum Link.
Fusarium sp.
Penicillium sp.
Phoma sp.
Phomopsis sp.
Pleospora herbarum (Fr.) Raben.
Rhodotorula rubra Harrison
Stemphylium sp.
SM I
SM II
Bacillus sp.
Average of growth stages
0a
0a
0a
1.33 a
0a
0a
0a
0a
0a
0a
0a
0a
0a
0a
0.67 a
0.67 a
0a
0a
0a
0a
0a
0a
0.12 aa
0.67 ab
0a
0a
0a
0a
2.67 abc
3.33 bc
0.67 ab
0a
3.33 bc
4.0 c
5.33 c
2 abc
0a
0a
0a
0a
9.33 d
0.67 ab
0.67 ab
0a
0.67 ab
1.52 b
14.0 d
0.67 ab
2.0 abc
0a
0.67 ab
2.0 abcd
2.0 abcd
0.67 ab
1.33 abc
7.33 f
1.33 abc
4.67 de
0a
0.67 ab
0a
0a
4 cde
6.67 ef
3.33 bcd
0a
1.33 abc
1.33 abc
2.45 c
4.89 de
0.22 a
0.67 a
0.44 a
0.22 a
1.56 ab
1.78 abc
0.44 a
0.44 a
3.56 cd
1.78 abc
3.33 bcd
0.67 a
0.22 a
1.33 a
0.22 a
0.22 a
5.33 e
1.33 a
0.44 a
0.22 a
0.67 a
Gs. 85: soft dough stage. *Growth stages according to Zadoks et al. (1974). Data are the mean of 150 leaf pieces
(5 pieces × 10 replications × 3 cultivars)/growth stage. Means followed by same letter in the same column are not
statistically different according to LSD (P ≤ 0.05). aFor the average of growth stages means followed by the same letter
in the same row are not statistically different (P ≤ 0.05). Gs.35: second node detectable. Gs.75: medium milk.
(P ≤ 0.05). A total of 1750 plant segments
were processed from wheat tissues and 33
microbes were recovered. Three bacteria, 27
fungal taxa and 3 non-sporulating mycelia,
assigned as ‘sterile mycelia’, were registered
(Tables 12.5 and 12.6). A. alternata, C. herbarum, E. nigrum, Cryptococcus sp., R.
rubra, Penicillium sp. and Fusarium
graminearum were the fungi that showed
the highest colonization frequency in all the
tissues and organs analysed. As is shown,
the bacterial isolates (Serratia sp., Bacillus
sp. and unidentified yellow bacteria) were
registered with high frequencies. The results
of this statistical analysis showed that
organs, microorganisms and interaction of
organs × microorganisms were significant.
On the other hand, as results of ANOVA
from each organ, we obtained that the number
of taxa isolated was greater in the leaves
than in the other organs analysed. Respectively, 25, 17, 12 and 15 were the number of
taxa recovered from leaves, stems, glumes
and grains. Few species were dominant
from grains, whereas they had the highest
percentages of isolates from the total samples analysed.
Likewise, a variation occurs in the species composition of endophytes isolated
from different organs and growth stages.
No significant differences between cultivars
were obtained, except when the glumes were
analysed. Whereas Bacillus sp. was isolated
from stems and grains, Serratia sp. and yellow bacteria were recovered from all organs
analysed.
Although most of the microorganisms
followed a similar pattern in the four organs,
Research in Endophytes from Agricultural Crops in Argentina
Table 12.5. Frequency (means) of microorganisms isolated from leaves, stems,
glumes and grains on five wheat cultivars.
Endophytes
Alternaria alternata (Fr.) Keissler
A. infectoria species group
Arthrinium sp.
Bacillus sp.
Bipolaris sorokiniana (Sacc.) Shoem.
B. spicifera (Bainier) Subramanian
Bipolaris sp.
Candida albicans (C.P. Robin) Berkhout
Cephalosporium sp.
Chaetomium globosum Kunze ex Fries
Cladosporium herbarum (Pers.:Fr.) Link.
Cryptococcus sp.
Cochliobolus spicifer Nelson
Curvularia lunata (Wakker) Boedijni
Epicoccum nigrum Link.
Fusarium oxysporum Schlechtend.: Fr.
F. graminearum Schwabe
Helicocephalum sp.
Nigrospora sp.
Penicillium sp.
Phoma sp.
Pleospora herbarum (Fr.) Raben.
Rhodotorula rubra Harrison
Septoria tritici Roberge in Desmaz.
Serratia sp.
Stachybotrys sp.
Stemphylium botryosum Wallr.
Trichoderma hamatum (Bonord.) Bainier
Ulocladium sp.
SM 1
SM 2
SM 3
Yellow bacteria
Organs
Leaves
Stems
Glumes
Grains
Cultivars
Klein Dragon
Buck Pronto
Klein Cobre
Buck Poncho
Pro INTA Federal
Means (all organs) and growth stages
8.48 e*
0.56 a
0.58 ab
1.26 ab
0.73 ab
0.00 a
0.26 a
0.04 a
0.06 a
0.19 a
6.55 d
2.14 b
0.14 a
0.01 a
4.38 c
0.53 a
1.01 ab
0.00 a
0.04 a
1.16 ab
0.00 a
0.00 a
1.27 ab
0.00 a
8.95 e
0.00 a
0.09 a
0.17 a
0.04 a
0.00 a
0.00 a
0.38 a
4.33 c
0.94 a
1.27 a
0.98 a
2.03 b
1.54 a
1.37 a
1.33 a
1.39 a
0.91 a
Note: *Means followed by the same letter in the same column within the same treatment are not
statistically different according LSD (P ≤ 0.05). SM, sterile mycelia.
157
158
S. Larrán and C. Mónaco
Table 12.6. Means of the frequencies of microorganisms, cultivars and growth stages for each organ
(leaves, stems, glumes and grains) of five wheat cultivars.
Endophytes
Leaves
Stems
Glumes
Grains
Alternaria alternata (Fr.) Keissler
A. infectoria species-group
Arthrinium sp.
Bacillus sp.
Bipolaris sorokiniana (Sacc.) Shoem.
B. spicifera (Bainier) Subramanian
Bipolaris sp.
Candida albicans (C.P. Robin) Berkhout
Cephalosporium sp.
Chaetomium globosum Kunze ex Fries
Cladosporium herbarum (Pers.: Fr.) Link.
Cryptococcus sp.
Cochliobolus spicifer Nelson
Curvularia lunata (Wakker) Boed. Boedijni
Epicoccum nigrum Link.
Fusarium oxysporum Schlechtend.: Fr.
F. graminearum Schwabe
Helicocephalum sp.
Nigrospora sp.
Penicillium sp.
Phoma sp.
Pleospora herbarum (Fr.) Raben.
Rhodotorula rubra Harrison
Septoria tritici Roberge in Desmaz.
Serratia sp.
Stachybotrys sp.
Stemphylium botryosum Wallr.
Trichoderma hamatum (Bonord.) Bainier
Ulocladium sp.
SM 1
SM 2
SM 3
Yellow bacteria
Cultivars
Klein Dragon
Buck Pronto
Klein Cobre
Buck Poncho
Pro INTA Federal
Growth stages
2
8
10.5
11.1
11.4
4.8 e*
1.4 abc
1.2 abc
0.0 a
1.0 abc
0.2 a
0.4 a
0.0 a
0.0 a
0.2 a
1.4 abc
2.4 cd
0.0 a
0.4 a
3.0 d
2.2 cd
1.0 abc
0.0 a
0.4 a
2.0 bcd
0.2 a
0.2 a
2.4 cd
0.2 a
3.3 d
0.2 a
0.2 a
0.6 ab
0.0 a
0.2 a
0.0 a
0.0 a
3.0 d
2.4 efg
0.0 a
0.16 ab
3.68 gh
0.0 a
0.0 a
0.48 abc
0.0 a
0.48 abc
0.0 a
1.28 abcde
4.8 h
0.0 a
0.0 a
2.88 fg
0.16 ab
2.88 fg
0.16 ab
0.0 a
2.08 def
0.0 a
0.0 a
3.04 fg
0.0 a
13.6 i
0.0 a
0.0 a
0.64 abcd
0.0 a
0.0 a
0.0 a
1.76 cdef
1.6 bcdef
9.33 e
0.26 ab
0.00 a
0.00 a
0.53 abc
0.00 a
0.00 a
0.00 a
0.00 a
0.00 a
2.13 cd
1.60 abc
0.00 a
0.26 ab
1.86 bc
0.00 a
0.00 a
0.00 a
0.00 a
0.80 abc
0.00 a
0.00 a
0.26 ab
0.00 a
12.53 f
0.00 a
0.00 a
0.26 ab
0.00 a
0.00 a
0.00 a
0.00 a
3.73 d
17.6 d
0.8 a
1.2 a
1.6 a
1.6 a
0.0 a
0.4 a
0.4 a
0.0 a
0.8 a
21.6 e
0.0 a
0.4 a
0.0 a
10.8 c
0.0 a
0.8 a
0.0 a
0.0 a
0.0 a
0.0 a
0.0 a
0.0 a
0.0 a
6.8 b
0.0 a
0.4 a
0.0 a
0.4 a
0.0 a
0.0 a
0.0 a
9.6 bc
0.90 a
1.09 a
0.83 a
1.12 a
0.97 a
1.21 a
1.01 a
1.40 a
1.26 a
1.48 a
1.53 c
0.89 ab
1.31 bc
1.05 abc
0.48 a
2.73 a
2.73 a
2.18 a
2.36 a
1.39 a
0.73 a
0.65 a
0.80 a
1.76 b
0.85 a
2.16 b
1.02 a
1.26 a
1.09 a
0.24 a
1.87 c
0.94 b
1.72 a
2.83 b
Note: *Means followed by the same letter in the same column within the same treatment are not statistically different
according to LSD (P ≤ 0.05). SM, sterile mycelia.
Research in Endophytes from Agricultural Crops in Argentina
there were some, A. alternata for example,
with higher values in grains and glumes
than in leaves and stems. The spectrum of
species isolated ranges from potential saprobes over taxa that probably are present
as natural symbionts to known pathogens
(Fisher et al., 1992). Whereas A. alternata,
C. herbarum and E. nigrum are species commonly abundant in the phylloplane and are
considered primary saprobes and minor
pathogens, others like B. sorokiniana, C.
lunata and F. graminearum are economically
important pathogens of wheat (Zillinsky,
1984).
Due to the fact that some of these endophytes adapted to a given organ may benefit
the host against pathogens, further studies
were undertaken.
A Biological Control Approach
to Infection of Drechslera
tritici-repentis in Wheat
The investigation was carried out to study
the interactions between some endophytes
isolated from healthy wheat plants and
Drechslera tritici-repentis and to determine
its possible significance in the biological
control of tan spot (Larran et al., unpublished). Endophytes isolated previously from
wheat cultivars in Buenos Aires Province,
Argentina, were selected for the assay. They
were: A. alternata, Bacillus sp., C. globosum,
C. herbarum, E. nigrum, Penicillium sp., R.
rubra, Trichoderma hamatum and P. lilacinus. Mycelial and conidial morphological
alterations and inhibition of colony growth
of D. tritici-repentis were registered under
in vitro conditions. Likewise, greenhouse
experiments were also carried out. The results
obtained from all tests have demonstrated
159
that endophytes may have a role as biocontrol agents against D. tritici-repentis.
Conclusions
The study of endophytes began with the
aim of studying their biodiversity and distribution from different hosts. We confirmed
that endophytes were present in all the
hosts evaluated. Then, we found that endophytes colonized distinct ecological niches
and could suggest their organ specificity
according to several authors (Sieber, 1988;
Fisher et al., 1991). On the other hand, in our
studies, we have isolated a large number of
species from healthy tissues of beet, tomato,
soybean and wheat but only few species
were dominant, in agreement with Petrini
et al. (1992). Distinct endophyte assemblages
were obtained from healthy tomato leaves
in 1998 and 1999, which could be explained
because of the different climatic conditions
prevailing in both years.
Endophytes could be adapted to their
hosts and be antagonists for their pathogens
and, depending on their antagonistic capacity, they would be able to displace, reduce,
suppress or induce resistance against them.
Nowadays, in accordance with the status of our investigation, we consider that
further studies are needed to evaluate the
possible use of endophytes as biocontrol
agents against pathogens of agricultural crops.
Intensive work is needed to understand the
role of endophytes and, mainly, their possible use as agents of biocontrol. Likewise,
it is very important to study the nature of
plant–endophyte–pathogen interactions and
the mechanism of antagonism (antibiosis,
hyperparasitism, competition) with the aim
of improving the efficiency of the biological
control of pathogens.
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13
Effect of Tillage Systems on the
Arbuscular Mycorrhizal Fungi
Propagule Bank in Soils
Santiago Schalamuk1,2 and Marta N. Cabello1,3
1Instituto
de Botánica ‘Spegazzini’; 2CONICET (Consejo Nacional de
Investigaciones Cientificas y Technicas), Universidad Nacional de La Plata,
La Plata, Argentina; 3CICBA (Comision de Investigaciones Cientificas
de la Provincia de Buenos Aires), Argentina
Abstract
In this chapter we discuss the effects of tillage and no-tillage systems on the characteristics of the
arbuscular mycorrhizal fungi (AMF) propagule bank in soils. These fungi, which belong to the phylum
Glomeromycota, are of great interest in agriculture. AMF are often assumed to be solely beneficial;
however, in certain environmental conditions, growth depressions related to AMF have been observed.
In soils under no-tillage, an intact hyphal network is present, whereas under conventional tillage, this
network can be damaged and AMF spores may remain as propagule sources. Some direct effects of
tillage on AMF propagules are: (i) disruption of the hyphal network; (ii) dilution of the propagule-rich
topsoil; and (iii) accelerated root decomposition. Spore counts in soils should be considered as useful
indicators for AMF activity in situ; however, the presence of spores does not always imply recent
activity of AMF and mechanical disturbance may change their spatial distribution in the soil profile.
Therefore, the information about spore numbers in agricultural systems needs to be analysed cautiously.
The different environmental conditions and direct effects related with tillage and no-tillage on AMF
communities generate shifts not only in the composition of the AMF soil propagule bank, but also in its
diversity. If the differential use of the various types of propagules by the Glomeromycota families, as
many authors suggest, is confirmed, the lack of disruption of the hyphal network in no-tillage can help
to explain the differences in Glomeromycota diversity that are found in field experiments.
Importance of AMF in Agriculture
Arbuscular mycorrhizae (AM) show symbioses between plant roots and fungi belonging to the phylum Glomeromycota (Schübler
et al., 2001). These fungi are obligate biotrophs and form associations with most plant
species (Trappe, 1987). AM associations are
the most frequent symbioses in nature because
of their broad association with plants and
162
their cosmopolitan distribution (Harley and
Smith, 1983). They have been found from
the Antartic Peninsula to the tropics (Huante
et al., 1993; Cabello et al., 1994). The wide
host range of these fungi and their ability to
grow in different environments are the reason
why arbuscular mycorrhizal fungi (AMF)
are usually considered ‘generalists’ with
low host specificity (Smith and Read, 1997).
Studies have confirmed that mycorrhizal
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Effect of Tillage Systems
fungi colonize most agricultural plants and
that they can have a substantial impact on
crop productivity (Johnson, 1993).
The interaction between the fungus and
its host plant consists mainly in nutrient
transfer: the plant provides the fungus with
carbon compounds, while the fungus delivers nutrients to the plant. The increased
nutrient uptake from the soil, particularly of
phosphorus and nitrogen, is the main benefit
attributed to mycorrhizal symbioses (Smith
and Read, 1997; Govindarajulu et al., 2005).
Other benefits may include enhancement of
resistance to root parasites (Borowicz, 2001),
improvement of drought tolerance (Augé,
2001) and reduction of the impact of environmental stresses such as salinity (RuizLozano et al., 1996). AMF also have an
important role in the improvement of soil
stability, which can possibly diminish erosion (Rillig et al., 2002).
AM fungi are often assumed to be solely
beneficial, since they are widely thought to
function as mutualists. However, their effects
on host growth often depend on environmental conditions such as nutrient availability and soil moisture (Peng et al., 1993;
Al-Karaki et al., 1998; Graham and Abbott,
2000; Valentine et al., 2001). As AMF draws
C from the host, the overall effect on host
growth depends on the cost–benefit relationship of the symbiosis (Johnson et al.,
1997; Grimoldi et al., 2005). Consequently,
in fertile soils, growth patterns of mycorrhizal plants often do not differ significantly
from those of non-mycorrhizal ones (Newsham et al., 1995) and even growth depressions related to AMF have been observed in
many plant species (Johnson et al., 1997;
Allen et al., 2003). In such plant–AMF interactions, only the fungal symbiont has a net
benefit, and this has sometimes been interpreted as parasitism (Johnson et al., 1997).
AM fungi are grouped into genera that
encompass more than 150 species described
to date and the effects that they have on their
host plants, or ‘effectivity’, differ greatly
between fungal strains or species (Miller et al.,
1985; Modjo and Hendrix, 1986). Since a single root can be colonized simultaneously by
various Glomeromycota species, AMF root
colonization is mediated by interspecific
163
fungal interactions, such as competition,
antagonism and dominance (Allen et al.,
2003). Because of the importance of AMF in
agrosystems, their study is relevant both for
the manipulation of indigenous AMF in the
field through appropriate agricultural practices and for the development of a successful inoculation.
Agricultural Practices
and Mycorrhizae
Agricultural practices for annual crops,
such as crop rotations, tillage, sowing, fertilization, pest, weed and disease control,
and harvest, generate changes that affect the
microbial communities in the rhizosphere.
Conventional tillage is characterized by the
use of disc or mouldboard ploughs, followed by harrowing for seedbed preparation. In no-tillage, seeds are drilled directly
into the soil with an appropriate planting
machine (Crovetto, 1992). No-tillage systems are characterized by the accumulation
of crop residues on the soil surface, leading
to greater carbon, nitrogen and surface water,
compared to conventional tillage (Doran and
Linn, 1994). Several changes in soil properties have been reported with no-tillage
management systems: improved aggregate
stability, moisture availability with residue
retention, changes in the distribution of
organic matter residues down the soil profile, for example, a more even distribution
of organic matter in cultivated soil as compared to that in non-tilled soil, where residues are concentrated on the surface
(Alvarez et al., 1998). One of the problems
that may occur in no-tillage is the nutritional deficiency because of the reduced
mineralization of the soil organic matter
(Fox and Bandel, 1986).
In the case of AMF, the lack of soil
physical disturbance in no-tillage might
wrongly suggest that soils with annual crops
under this system may be similar to those of
natural grasslands. However, agroecosystems have particular characteristics which
influence AMF activity. Natural ecosystems
present various plant species hosting AMF,
164
S. Schalamuk and M.N. Cabello
at different phenological stages. Annual crops,
however, inherently represent a change for
AMF, because of the reduction in host
biodiversity. In addition, cropped systems
show two clearly different periods: a period
with high density of host plants of the same
species growing simultaneously and, after
harvesting, the fallow period with no host
or, in some cases, scarce presence of spontaneous vegetation (i.e. weeds). As obligate
symbionts, Glomeromycota relies on the
plant host for the supply of C assimilates
required for its growth, maintenance and
functioning. Therefore, dynamics and biodiversity are clearly affected by agricultural
practices (Kurle and Pfleger, 1994).
Significance of the AMF Propagule
Bank on Root Colonization
Effect of tillage
Colonization of roots by AM fungi can arise
from three sources of inoculum: spores, colonized root fragments and hyphae. The
propagules in soils therefore may be called
a ‘propagule bank’ that is ‘waiting’ for suitable conditions to germinate, grow and
eventually colonize new plant roots (Öpik,
2004; Schalamuk, 2005). Most of the host
plant benefits obtained by AM symbiosis,
mainly phosphorus acquisition, depend on
the early colonization of roots. The rapid
colonization is related to AMF propagule
density and composition, i.e. the so-called
propagule bank. A graph of the percentage
of the root length colonized against time has
a sigmoid form showing three phases: lag
phase, linear phase and a plateau (Sieverding, 1991). A higher AMF propagule density
often reduces the length of the lag phase and
thereby accelerates the process of mycorrhizal colonization (Smith and Read, 1997).
Numerous studies have shown that
mycorrhizal colonization is affected negatively by tillage (Douds et al., 1995; McGonigle and Miller; 1996a; Kabir et al., 1998;
Mozafar et al., 2000). Soil disturbance
reduces AMF propagule density since tillage of soil breaks up the AM fungi hyphal
network and consequently lowers mycorrhizal colonization (McGonigle and Miller,
1996a). At the final crop stages, the AMF
colonization levels in no-tillage and conventional tillage often do not differ significantly; however, at the early stages, crop
plants under no-tillage often show higher
mycorrhizal colonization (Schalamuk et al.,
2004). As already mentioned, in no-tillage
systems, the reduced mineralization of the
soil organic matter often generates plant
nutritional deficiencies. Nevertheless, a
higher nutrient concentration related to a
rapid AMF colonization has been observed
under no-tillage systems (McGonigle and
Miller, 1996a; Mozafar et al., 2000; Schalamuk et al., 2004). By using the method of
Plenchette et al. (1989), we have previously
found higher levels of mycorrhizal soil
infectivity in no-tillage systems (Schalamuk
et al., 2004). As already pointed out, colonization of roots by AM fungi can arise from
different sources of inoculum. Colonized
root fragments (Rives et al., 1980), spores
(Gould and Liberta, 1981; Jasper et al., 1987,
1988) and hyphae (Jasper et al., 1989) lose
their ability to initiate colonization with
soil disturbance, which can be related to
physical damage to the propagules by tillage and/or unfavourable conditions for germination or colonization after disturbance
(Stahl et al., 1988; Bellgard, 1993).
Mycorrhizal soil infectivity (MSI)
(Plenchette et al., 1989) compares the ability of different soils to induce colonization
in plants and depends on the activity of all
the propagule types in soil. It is difficult to
distinguish the relative contributions of the
different types of propagules to the colonization of root systems (Smith and Read,
1997), and mycorrhizal infectivity does not
provide information about the relevance of
each propagule type in any particular field
situation. Although a number of different
propagule types exist in the soil, they may
not be equally effective at producing new
infection units (Klironomos and Hart, 2002).
In many habitats, the hyphal network in the
soil, together with root fragments, is probably the main means by which plants become
colonized, even when significant spore
populations are also present (Hepper, 1981;
Effect of Tillage Systems
Tommerup and Abbott, 1981; Birch, 1986;
Jasper et al., 1992). Studies have shown that
AMF extraradical hyphae are affected severely
by soil disturbance at tillage (Fairchild and
Miller, 1990; McGonigle and Miller, 1996b;
Kabir et al., 1997; Wright and Upadhyaya,
1998). Jasper et al. (1989) have stated that
due to the importance of the AMF hyphal
network as inoculum in undisturbed soil, a
lower infectivity of soil propagules after the
disturbance usually can be determined by
the damage on the network, rather than on
spores and colonized root fragments. Another
effect of tillage on the AMF propagule bank,
which occurs simultaneously with the disruption of the hyphal network, is the dilution of the topsoil rich in propagules, with
the poorest part in the subsurface (Sieverding, 1991). Clearly, mechanical soil mixing
affects all types of AMF propagules.
As a conclusion, it is suggested that tillage affects all types of AMF propagules
directly, to a greater or lesser extent, through
different mechanisms acting together: (i)
disruption of the hyphal network; (ii) dilution of the propagule-rich topsoil; and (iii)
accelerated root decomposition. Through
all these direct effects, tillage may reduce
soil mycorrhizal infectivity and thereby AM
root colonization at the early stages of crop
growth.
Effects of Tillage and Cropping on
AMF Spore Densities in Soils
AMF spores are formed by differentiation of
vegetative hyphae in soil or roots and appear
to be long-term survival structures. In agricultural systems with annual crops, other
propagule types (i.e. hyphae inside and outside the roots) seem to be more important to
start colonization in particular conditions.
Nevertheless, spore counts in soils should
be considered as useful indicators for
AMF activity in situ. Several studies have
found higher spore numbers in no-tillage
than in conventional tillage (Crovetto, 1985;
Kabir et al., 1998; Jansa et al., 2002; Schalamuk et al., 2003). In agroecosystems with
annual crops, the number of spores generally
165
increases during the growing cycle (Cabello,
1987) and sporulation is frequently linked
to host phenology in the field (e.g. maximum spore production occurs near the middle or the end of a growing season) (Morton
et al., 2004). At the early stages of the crop,
higher spore densities are usually found in
untilled soils, in comparison with conventional systems, whereas at the more advanced
phenological stages, differences between
tillage systems are reduced (Schalamuk et al.,
2003).
It is well known that spores can survive
in soils for several years (Sieverding, 1991).
Thus, spore counts reflect both the sporulation and the action of many factors that
affect their survival and accumulation in
the soil. Consequently, spore density is a
result of a complex balance and, while sporulation is probably related to the recent
activity of the AMF, spore counts in the soil
include structures formed at different times.
Spore production depends on carbon
supply from the host to the fungus (Furlan
and Fortin, 1977; Daft and El Giahmi, 1978).
Douds et al. (1993) have indicated that the
production of fungal AM spores can decrease
when soils are tilled. Increases in spore numbers have been associated with root growth
(Hayman, 1970) and/or with host maturity or
senescence (Hayman, 1970; Koske and Halvorson, 1981; Giovannetti, 1985; Gemma
et al., 1989; Troeh and Loynachan, 2003).
Agricultural practices generate disturbances
that affect AMF colonization and, in turn,
spore formation in soils (Kurle and Pfleger,
1994). Therefore, tillage, either through
changes in mycorrhizal colonization or
through indirect effects, such as changes in
the soil environment and plant growth, largely
affect AMF spore production in soils.
The survival of a spore depends on its
morphological traits, determined mainly by
the species of Glomeromycota to which it
belongs, as well as on the characteristics of
the soil environment. Spore survival is an
important factor determining the variations
in AMF spore counts in soils; however,
information about spore survival is scarce
as compared to that about sporulation (Lee
and Koske, 1994a). In natural ecosystems,
decreases in spore numbers have been
166
S. Schalamuk and M.N. Cabello
attributed mainly to their germination, the
activity of macro and micro fauna and their
destruction by other soil fungi and parasites
(Gerdemann and Trappe, 1974; McIlveen
and Cole, 1976; Ross and Ruttencutter, 1977;
Ross and Daniels, 1982; Rabatin and Stinner, 1985, 1988). AMF spores are commonly
infected either by other fungi (Daniels and
Menge, 1980; Lee and Koske, 1994a; Rousseau et al., 1996) or by actinomycetes (Lee
and Koske, 1994b), and environmental conditions have a strong influence on these
processes (Janos, 1980; Koske, 1988). In
agricultural systems, another effect that
directly reduces spore counts is the dilution of the topsoil rich in spores with the
part in the subsurface poorer in propagules
(Crovetto, 1985; Sieverding, 1991). For all
these reasons, spore survival and accumulation may have a great influence on spore
counts, and the largest spore numbers in notillage at the early stages may be the result
of either higher or faster sporulation and/or
the presence of residual spores produced
during the fallow or the previous crop. As
the presence of spores does not always
imply recent activity of AMF, and mechanical disturbance may change their spatial
distribution in the soil profile, the information about spore numbers in agricultural
systems is useful, but needs to be analysed
cautiously.
AMF Propagule Bank and
Biodiversity
As already pointed out, tillage may alter the
AMF propagule bank in several ways and
the lack of disturbance in continuous notillage systems can generate accumulative
effects. Therefore, in soils under no-tillage,
an intact hyphal network can be present,
whereas under conventional tillage, this network can be damaged and AMF spores may
remain as propagule sources. Little information exists on the effect of tillage systems on
Glomeromycota diversity (Jansa et al., 2002;
Schalamuk et al., 2006). Several studies
have shown that Glomeromycota taxa may
vary in their colonization strategies and that
these variations can be associated with the
utilization of different propagule types by
AMF families (i.e. Acaulosporaceae, Gigasporaceae and Glomeraceae) (Tommerup and
Abbott, 1981; Biermann and Linderman,
1983; INVAM, 1993; Braunberger et al., 1996;
Brundrett et al., 1999; Klironomos and Hart,
2002; Hart and Reader, 2002, 2004). Jansa
et al. (2002), in an intensively used agricultural soil under long-term reduced tillage
management, found that the presence of
certain AMF species, especially those that
did not belong to Glomus spp., had a tendency to increase. However, we have found
that the contribution of species belonging to
the Glomeraceae family increases in notillage plots, to the detriment of Acaulosporaceae and Gigasporaceae (Schalamuk et al.,
2006). In that experiment, the greatest contribution of Glomeraceae species in no-tillage
indicated a lower equitability in the distribution among the families of Glomeromycota,
and thereby a lower diversity, in comparison with conventional tillage. These findings differ from those of Jansa et al. (2002).
Nevertheless, it is important to point out
that mycorrhizal communities are sitespecific and that each AMF species can be
affected in several ways by different agricultural management practices; therefore, generalization is difficult.
De Souza (2005), based on life history
strategy studies, suggested that members
of the Gigasporaceae family were ‘K’ strategists in contrast to single spore-producing
‘Glomus’ species. Hart and Reader (2004)
found that the Gigasporaceae family was
less sensitive to soil disturbance than the
Glomeraceae. The basis for this difference
between both families is due probably to
differences in their colonization strategies. AM fungi in the Gigasporaceae colonize primarily from spores, whereas those
belonging to the Glomeraceae can colonize from hyphae (Tommerup and Abbot,
1981; Biermann and Lindermann, 1983).
Hyphae are more sensitive to soil disturbance than spores and thus subsequent
colonization of additional roots is affected
more.
Tillage or the lack of disturbance in
continuous no-tillage determine different
Effect of Tillage Systems
environmental conditions and direct effects
on AMF communities, and thereby shifts in
the composition of the AMF soil propagule
bank. Consequently, if the differential use
of the various types of propagules by the
Glomeromycota families, as many authors
suggest, is confirmed, the lack of disruption
of the hyphal network in no-tillage for a
period of several years can help to explain
the higher proportions of Glomeraceae that
have been found previously in the system
(Schalamuk et al., 2006).
167
Conclusions
Tillage and continuous no-tillage systems
change the composition of the AMF propagule
banks in the soil, whereas mechanical soil
mixing affects all types of AMF propagules.
Continuous no-tillage systems favour the
presence of an intact hyphal network in
soils. Possible differences in colonization
strategies among Glomeromycota taxa might
have a great influence on the impacts of tillage on AMF diversity.
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14
Mechanism of Action in Arbuscular
Mycorrhizal Symbionts to Control
Fungal Diseases
Arun Arya, Chitra Arya and Renu Misra
Department of Botany, Faculty of Science, The Maharaja Sayajirao University
of Baroda, Vadodara, India
Abstract
Currently, the world over, especially in developing countries, maintenance of soil fertility and control
of plant diseases have become crucial issues in meeting the biomass needs for food, fodder and fuel,
as well as preserving a clean environment. An ideal fertile soil is characterized not only by optimum
physical properties and chemical constituents conducive for plant growth, but also by microbiological
processes that are maintained in equilibrium. More than 90% of land plants are estimated to form
arbuscular mycorrhizal (AM) associations with soilborne fungi in the phylum Glomeromycota. They
have a wide host range, yet certain host and fungal combinations are more effective from either the
perspective of the fungus, i.e. greater spore/hyphae production, or from that of the host, i.e. enhanced
growth, nutrient acquisition or pathogen resistance. Besides improving uptake of phosphorus, AM
fungi improve plant health through improved resistance to various biotic and abiotic stresses. Of particular importance is the bioprotection conferred to plants against many soilborne pathogens, such as
species of Aphanomyces, Cylindrocladium, Fusarium, Macrophomina, Phytophthora, Pythium,
Rhizoctonia, Sclerotium, Thielaviopsis and Verticillium, as well as various nematodes by AM fungal
colonization of the plant roots.
Achieving the effective and sustainable control of plant diseases remains a formidable challenge
for all agricultural systems. Despite the continued release of resistant cultivars and pesticides, pathogens still cause crop damages and losses that exceed 12% worldwide. Studies have shown that root rot
in wheat caused by S. rolfsii was prevented by the inoculation of Glomus fasciculatum. Reduced quantum of lesioned roots was found in take-all diseases caused by Gaeumannomyces graminis tritici due
to G. deserticola in wheat. The association of G. radiatum with apple has been studied in the USA. It
was found that soilborne fungi, Cylindrocarpon, Pythium and the parasitic nematode, Pratylenchus
spp., were common with replant diseases of apple. In this disease, young trees are stunted and develop
fewer branches than healthy trees.
The exact mechanisms by which AM fungal colonization confers the protective effect are not
completely understood, but a greater understanding of these beneficial interactions is necessary for the
exploitation of AM fungi in organic and/or sustainable farming systems. The mechanisms employed
by AM fungi indirectly to suppress plant pathogens include enhanced nutrition to plants; morphological changes in the root; increased lignification; changes in the chemical composition of the plant
tissues like antifungal chitinases, isoflavonoids, etc.; alleviation of abiotic stress and changes in the
microbial composition in the mycorrhizosphere. Bioprotection within AM fungal-colonized plants is
the outcome of complex interactions between plants, pathogens and AM fungi. In this chapter, the
different diseases of cereals, pulses, fruits and vegetables and the potential mechanisms by which AM
fungi contribute to bioprotection against plant soilborne pathogens are discussed.
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
171
172
A. Arya et al.
Introduction
Arbuscular mycorrhizal (AM) symbiosis is
the most commonly occurring underground
symbiosis in plants. It can be found in a
large majority of terrestrial plants (Newman
and Reddell, 1987) and in almost a quarter
of a million plant species. It is as normal for
the roots of plants to be mycorrhizal as it is
for the leaves to photosynthesize (Mosse,
1986). The AM fungi are included in the phylum Zygomycota, order Glomales (Redecker
et al., 2000), but recently they have been
classified into the phylum Glomeromycota
(Schussler et al., 2001). The phylum is
divided into 4 orders, 8 families, 10 genera
and 150 species; the common genera are
Aculospora, Gigaspora, Glomus and Scutellospora (Schussler, 2005). They are characterized by the presence of extra-radical
mycelium, branched haustoria-like structures within the cortical cells, termed arbuscules. These are the main sites of nutrient
transfer between the two symbiotic partners
(Hock and Verma, 1995; Smith and Read,
1997).
AM fungi colonize plant roots and penetrate the surrounding soil, extending the
root depletion zone and the root system.
They supply water and mineral nutrients
from the soil to the plant, while AM benefit
from carbon compounds provided by the
host plant (Smith and Read, 1997). AM
fungi are associated with improved growth
of host plant species due to increased nutrient uptake, production of growth-promoting
substances, tolerance to drought, salinity
and synergistic interactions with other
beneficial microorganisms (Sreenivasa and
Bagyaraj, 1989). The beneficial role of AM
fungi in plant biomass production is associated with their capacity to reduce or prevent
the development of plant disease (Manoharachary, 2004). The protective ability of
mycorrhizae is generally observed against
soilborne diseases and is often related to the
nature of the host plant, mycorrhizal symbionts, plant pathogens and the condition of
the soil (Tello et al., 1987). AM fungi are
helpful in controlling disease; however, Ross
(1972) reported increased development of
Phytophthora root rot in soybean. The
known interaction may include a number
of mechanisms such as exclusion of pathogens, lignification of plant cell wall and
change in phosphorus nutrition, leading to
exudation by roots and the formation of
inhibitory low molecular weight compounds. The mycorrhizal fungi can produce certain compounds that inhibit or kill
the pathogenic fungi.
Interaction of AM Fungi
with Fungal Pathogens
Cereal crops
Achieving the effective and sustainable control of plant disease remains a formidable
challenge for all agricultural systems. Despite
the continued release of resistant cultivars
and pesticides, pathogens still cause crop
damages and losses that exceed 12% worldwide (Johar, 2005). Root rot in wheat caused
by S. rolfsii was prevented by inoculation of
G. fasciculatum (Harlapur et al., 1990). Graham and Menge (1982) reported reduced
quantum of lesioned roots in take-all disease caused by G. graminis tritici due to
G. deserticola in wheat.
It was found that root dry weight of
paddy was not affected by R. solani in
mycorrhizal plants, but the pathogen
caused 29% loss in root dry weight in nonmycorrhizal plants (Khadge et al., 1990).
Also, the pathogen multiplied less in mycorrhizal plants. Cochliobolus sativus negated
the effect of VAM inoculation in locally
adapted WI 2291 cultivar of barley, whereas
in the absence of the pathogen, AM inoculation increased grain yield from 31.9 g to
46.6 g in phosphorus fertilized plants but
did not have fertilized plants (Grey et al.,
1989). Contrary results were obtained by
Schonbeck and Dehne (1979), who observed
increase in disease due to Erysiphe graminis
and Helminthosporium sativum in barley.
The severity of common root caused by
Bipolaris sorokiniana in barley was reduced
by three species of Glomus (Boyethko and
Tewari, 1990).
Mechanism of Action to Control Fungal Diseases
Pulses and oil crops
Gigaspora calospora exerted an inhibitory
effect on the development of pigeon pea
blight caused by P. drechsleri f. sp. cajani
(Bisht et al., 1985). Similarly, in Tamil Nadu
Agricultural University, India, studies showed
that another AM fungus, G. etunicatum,
induced tolerance to cowpea (Vigna unguiculata) against Macrophomina root rot. Disease incidence was 16% in inoculated plants
as against 33% in uninoculated plants (Ramraj et al., 1988). Rosendahl (1985) observed a
decrease in disease incidence in peas due
to Aphanomyces euteiches. Similar results
were observed for soybean (Zambolin and
Schenck, 1983) and groundnut (Abdalla and
Abdel-Fattah, 2000) due to F. solani. Krishna
and Bagyaraj (1983) observed a reduction in
disease due to M. phaseolina in soybean.
Studies conducted at the University of Bayreuth, Germany, showed that in leachates of
AM rhizospheric soil of Zea mays and Trifolium subterraneum, fewer sporangia and
zoospores were produced by P. cinnamomi
as compared to non-AM plants, suggesting
that sporangium-induced microorganisms
declined or sporangium inhibitors increased
(Meyer and Linderman, 1983).
Pandey and Upadhyay (2000) studied
the effect of microbial populations on the
development of pigeon pea in Pusa, Bihar,
India. Screening for resident antagonists
was carried out and the mode of mycoparasitism was studied. Dual inoculation with
AM endophyte (G. mosseae) and M. phaseolina restricted the progression of the pathogen significantly in the roots of mungbean
(V. radiata). Disease incidence was reduced
from 77.9% in pathogen inoculated to 13.3%
in AM + pathogen inoculated plants (Jalali
et al., 1990). G. fasciculatum reduced the
number of sclerotia produced by S. rolfsii in
groundnuts (Arachis hypogaea) (Krishna
and Bagyaraj, 1983).
Horticultural crops
The early wilt symptoms caused by F.
oxysporum on tomato appeared 8–10 days
173
earlier in mycorrhizal plants. However,
2 months later, disease severity was reduced
significantly in these plants. Between the
two species tested, G. etunicatum was more
effective than G. mosseae (Sharma and Johri,
2002). Brassica oleracea infected with AM
fungus had lower infection by R. solani;
higher moisture content (25%) enhanced
disease incidence (Iqbal et al., 1988). Studies conducted at the University of Jordan,
Jordan, showed that the mycorrhizal plants
of tomato inoculated with F. oxysporum
had significantly higher root and shoot
weights and plant heights than plants inoculated with F. oxysporum only (Al-Momany
and Al-Radded, 1988). Only the presence
of G. intraradices resulted in a significant
decrease in the population of F. oxysporum
and root necrosis (Caron et al., 1986).
Early infestation of G. fasciculatum enhanced
tomato plant growth and reduced Fusarium
wilt (Manian et al., 2006). They also observed
that the percentage disease index was less
in mycorrhizal than in non-mycorrhizal
tomato plants when inoculated with Alternaria solani.
The presence of G. mosseae decreased
both weight reduction and root necrosis in
tomato caused by P. nicotianae var. parasitica (Trotta et al., 1996). In vitro experiments in which Ri T-DNA transformed
roots of alfalfa were inoculated with AM
fungi showed normal mycorrhizal formation
by G. intraradices and hypersensitivity-like
response to G. margarita. Colonized cells
became necrotic and HPLC studies indicated
concentration of phenolics and isoflavonoids in these roots. The data strongly
support the existence of a degree of specificity between AM fungi and the host (Douds
et al., 1998).
Onion pink rot caused by Pyrenochaeta
terrestris and tomato root rot caused by T.
basicola are controlled by mycorrhizal
fungi (Vidhyasekaran, 2004). Inoculation of
G. mosseae in tomato and eggplant seedlings controlled the incidence of Verticillium wilt caused by V. dahliae in Greece
(Karagiannidis et al., 2002). Trotta et al.
(1996) studied the interaction between the
soilborne root pathogen P. nicotinae var.
parasitica and the arbuscular mycorrhizal
174
A. Arya et al.
fungus G. mosseae in tomato plants. Treatment with Phytophthora resulted in a visible
reduction in plant weight and in a widespread root necrosis in plants without mycorrhiza. The presence of AM fungus decreased
both weight reduction and root necrosis.
The percentage reduction of root necrosis
ranged between 63 and 89%.
Utkhede et al. (1992) studied the effect
of G. mosseae on replant disease of apple. It
was found by Graham and Egel (1988) in
Florida, USA, that G. intraradices did not
increase the resistance or tolerance of sweet
orange seedlings to Phytophthora root rot
unless mycorrhizae conferred a phosphorus nutritional advantage over the nonmycorrhizal plants. Citrus root rot caused
by P. parasitica and T. basicola can be controlled by AM fungi (Vidhyasekaran, 2004).
Prior root colonization by mycorrhizal fungi,
G. margarita or G. macrocaropum, reduced
the damage caused by P. parasitica in two
citrus root stocks, Carrigo citrage and Sour
orange (Schenck et al., 1977). To ensure
good mycorrhizal establishment in citrus
roots, plants were exposed for 110 days to
mycorrhizal fungi before challenging them
with the pathogen. In phalsa (Grewia subinaequalis), better root growth and feeding
sites of nematodes during the rainy season
promoted better colonization of AM fungi
(Hasan and Khan, 2006).
Cash crops
Studies conducted at the Rajasthan Agriculture University, India, showed that Cuminum
cyminum in association with G. calospora, G.
fasciculatum, G. mosseae and Acaulospora
laevis enhanced nutrient uptake and reduced
wilt severity due to F. oxysporum f. sp.
cumini (Champawat, 1991). In Germany, G.
etunicatum reduced leaf blight in rubber
plants caused by Microcycles ulei (Feldmann et al., 1990). G. monosporum inoculated tobacco plants showed better tolerance
against T. basicola (Giovannetti et al., 1991).
Sivaprasad et al. (2006) controlled foot rot
of black pepper by inoculation of G. monosporum. Two other species, G. etunicatum
and G. mosseae, exhibited a medium level
of resistance to the disesases. Rhizome rot
of ginger caused by P. aphanidermatum was
controlled by G. mosseae and G. fasciculatum (Sivaprasad et al., 2006).
Field application of a commercially
available formulation of AM marketed as Josh
by Cadila Pharmaceuticals, Agro Division,
was tried for the management of charcoal
stump rot disease caused by Ustulina zonata
(Chakraborty et al., 2005). Commercial production of the medicinal plants in arid and
semi-arid areas of the Thar Desert is affected
mostly by the soilborne plant pathogens
ready to attack any seedlings transplanted
into the field. Mycorrhizal symbiosis resulted
in significant disease severity in Chlorophytum
borivillianum, Convolvulus microphyllous
and Withania somnifera (Vyas, 2005).
Role of AM fungi in forestry
Studies conducted at the Northern Forest
Research Centre, Canada, showed that Fusarium wilt disease severity in Albizia procera
and Dalbergia sissoo was reduced significantly when inoculated with mycorrhizal
fungi (Chakravarty and Mishra, 1986). The
effect of AM fungi, Pseudomonas and Rhizobium, was observed on the rate of photosynthesis and colonization in D. sissoo (Bisht
et al., 2006). The rate of photosynthesis was
significantly higher in plants inoculated
with AM consortium. Arya and Chaterjee
(1995–1996) found better plant biomass and
good growth of neem seedlings after inoculation of G. fasciculatum. Arya (2006) recorded
a change in soil mycoflora after inoculation
of AM fungus in neem seedlings. Fungi
like Aspergillus fumigatus, A. nidulans, A.
ochraecous and F. pallidoroseum were not
recorded after 3 months.
A significant increase in dry weight of
Santalum (Krishnamurthy et al., 1998) and
Tamarindus (Bagyaraj and Reena, 1990) seedlings has been observed after inoculation of
AM fungi. In ectomycorrhizae, the presence
of a mantle around the root prevents the entry
of pathogens, while in endomycorrhizae, the
better nutrient uptake makes the plant more
resistant to various pathogens.
Mechanism of Action to Control Fungal Diseases
Fungi are harmful agents to humans but
mycorrhizal fungi are indispensable for luxuriant growth of forest trees. Contrary to
popular belief, the luxuriance of rainforest
is not because the rainforest soil is more fertile (as torrential rains over millennia leach
out soluble minerals), but because the roots
associate with fungi, whose spreading hyphae
increase the area of absorption of scarce nutrients and transport this to the plant in return
for photosynthetically fixed carbon (Maheshwari, 2005). In Ghana and the Mopri Forest
Reserve of Cote d’Ivoire, Terminalia ivorensis plantations are susceptible to dieback,
the cause of which is unknown; poor mycorrhizal infection may be a contributory
factor (Wilson et al., 1994).
Signalling Pathway in Mycorrhiza
The signalling pathway to activate the
mycorrhiza-specific phosphate transporter
has its origin in the PL (phospholipid) PC
(phosphatidylcholine), imager component
of membranes of plants and probably, also
of the AM fungus. However, PC is not active
in itself. It gains activity only after treatment
with PLA2 and PC from plants, fungus or
both remains to be explored further. Several
PLA2s have been identified in plants and all
are secretory proteins. Their regulation and
substrate specificity are unknown. This
might hint at extracellular production of the
LPC (lyso-phosphatidylcholine) signal might
be generated more specifically in the arbuscules containing cells. LPCs are highly mobile
within the intact cells and LPC is therefore
a good candidate for a cytoplasmic messenger that transduces signals to activate downstream processes and gene expression in the
nucleus (Drissner et al., 2007).
Bioprotectant Nature of AM Fungi
Plant diseases can be controlled by manipulation of indigenous microbes or by
introducing antagonists to reduce the diseaseproducing propagules (Linderman, 1992).
AM fungi and their associated interactions
175
with plants reduce the damage caused by
plant pathogens (Harrier and Watson, 2004).
These interactions have been documented
for many plant species. With the increasing
cost of inorganic fertilizers and the environmental and public health hazards associated with pesticides and pathogens resistant
to chemical pesticides, AM fungi may provide a more suitable and environmentally
acceptable alternative for sustainable agriculture (Table 14.1).
Mechanism of Disease Control
Any one or more mechanisms may be operative in plants, imparting them with resistance against pathogens.
1.
2.
3.
Physical alteration in plant body.
Physiological changes.
Biochemical mechanisms
Physical alteration in plant body
According to some scientists, AM affects
soilborne plant pathogens on the basis of
physical alterations. Lignification of cell wall
and production of other polysaccharides has
been reported, which prevents penetration
of mycorrhizal plants by F. oxysporum
(Dehne and Schonbeck, 1979) and Phoma
terrestris (Becker, 1976). Mycorrhizal inoculation improves plant growth. Arya (2006)
found better growth of neem seedlings after
inoculation with three isolates of G. fasciculatum. It has also been suggested that a
stronger vascular system of the mycorrhizal
plants is likely to increase the flow of nutrients, impart greater mechanical strength and
diminish the effect of vascular pathogens
(Schonbeck, 1979). A few electron opaque
structures resembling the deposits were
found in some cells and intercellular spaces
of non-infected mycorrhizal carrot roots, but
were absent in infected, non-mycorrhizal
carrot roots. Restriction of pathogen growth,
together with an increase in hyphal alteration and accumulation of new plant products in mycorrhizal roots, but absent in
176
A. Arya et al.
Table 14.1. Effects of AM fungi on fungal diseases of certain crops.
Crop
AM fungi
Pathogen
Reference
Tomato
Glomus intraradices
Fusarium oxysporum f.
sp. lycopersici
F. oxysporum
Caron et al., 1986; Akkopru
and Demir, 2005
Al-Momany and Al-Raddad,
1988
Bhagawati et al., 2000
Pozo et al., 2002
Berta et al., 2005
Declerck et al., 2002
Declerck et al., 2002
Yao et al., 2002
Rosendahl and Rosendahl,
1990
Hao et al., 2005
Al-Momany and Al-Raddad,
1988
Ozgonen and Erkilic, 2007
Torres-Barragan et al., 1996
Rosendahl, 1985
Devi and Goswami, 1992
Sundaresan et al., 1993
Siddiqui and Singh, 2004
Akhtar and Siddiqui, 2006
Akhtar and Siddiqui, 2007
Liu, 1995
G. mosseae
Banana
Cucumber
Pepper
Onion
Pea
Cowpea
Chickpea
Cotton
G. etunicatum
G. mosseae
G. intraradices
Glomus sp.
G. proliferum
G. etunicatum
G. etunicatum
Glomus sp.
G. etunicatum
G. mosseae
Glomus sp.
G. fasciculatum
G. fasciculatum
G. fasciculatum
G. fasciculatum
G. intraradices
G. fasciculatum
G. mossae
G. vesiformae
F. oxysporum f. sp. lycopersici
Phytophthora parasitica
Rhizoctonia solani
Cylindrocladium
Spathiphylli
R. solani
Pythium ultimum
F. oxysporum f. sp. cucumerinum
F. oxysporum
P. capsici
Sclerotium cepivorum
Aphanomyces euteiches
Macrophomina phaseolina
F. oxysporum
F. oxysporum f. sp. ciceris
M. phaseolina
M. phaseolina
V. dahliae
non-mycorrhizal roots, shows that mycorrhizal infection is responsible at least in
part for the plant defence system which provides protection against pathogen attack
(Benhamon et al., 1994).
Physiological changes
AM fungi can interact directly with the
pathogens through phenomen like antagonism, antibiosis or predation. The studies
conducted so far suggest that they affect
the host–pathogen relationship indirectly
through physiological alteration or by competing for space or host resources. Through
increased P nutrition, AM fungi enhance
root growth, expand the absorptive capacity
of the root system for nutrients and water
and affect cellular processes in roots (Hussey
and Roncadori, 1982; Reid et al., 1984;
Smith and Gianinazzi, 1988). In addition to
phosphorus, AM fungi are known to enhance
uptake of Ca, Cu, S and Zn (Gerdemann,
1968; Sharma, 1990). Glomus monosporum
was found effective against P. capsici in
black pepper (Sivaprasad et al., 2006). The
authors found resistance due to improved
nutrient uptake. Host susceptibility to infection by the pathogen and tolerance to disease is influenced by the nutritional status
of the host and the fertility status of the soil
(Wallace, 1973). For example, nematodedamaged plants frequently show deficiencies of B, N, Fe, Mg and Zn (Good, 1968).
High levels of P fertilization in the absence
of AM fungi can interact with minor elements, creating a deficiency situation which
predisposes plants to root knot nematodes
(Smith et al., 1986). AM fungi may, therefore,
also increase host tolerance to pathogens
by increasing uptake of essential nutrients
other than P which are otherwise deficient
in non-mycorrhizal plants. Production of
Mechanism of Action to Control Fungal Diseases
siderophore can suppress root pathogens
(Sharma and Johri, 2002). Higher levels of
amino acids, especially arginine, in combination with root exudates of the mycorrhizal plant have been reported to reduce
chlamydospore production of T. basicola
(Baltruschat and Schoenbeck, 1975). Increased levels of phenylalanine and serine have
been observed in tomato roots inoculated
with G. fasciculatum. High concentrations
of orthodihydroxy (O-D) phenols in mycorrhizal roots suppressed the growth of S.
rolfsii (Goodman et al., 1967; Krishna and
Bagyaraj, 1983). The presence of HCN precursors has been observed in rubber plant
infected with G. etunicatum (Lieberei and
Feldmann, 1990).
Biochemical mechanisms
The production of phytoalexins in AMcontaining plants has been demonstrated
conclusively. Enhanced accumulation of
glyceollin I, a highly antifungal phytoalexin,
has been reported in the roots of mycorrhizal
soybeans (Morandi et al., 1984). According
to Sharma and Johri (2002), it is not clearly
understood how AM fungi induce the production of phytoalexins and elicitors. It may
be possible that mycorrhizal fungi perturb
root tissues so that the plant elicitors are liberated. Cell damage, which is closely associated with the production of isoflavanoids
in legumes (Bailey, 1982), has been observed
rarely in mycorrhizal soybean roots. The
concentration of coumestrol increased in
mycorrhizal roots (25 µg/g) and was much
greater than that of glyceollin I (Morandi
et al., 1984); coumestrol inhibits the growth
of bacteria and nematodes.
According to Chakraborty et al. (2005),
induction of disease resistance in pea
plants against charcoal stump rot was associated with the accumulation of defence
enzymes, followed by stimulation of antifungal phenolics.
Roots colonized by an AM fungi exhibit
high chitinolytic activities. These enzymes
can be effective against other fungal pathogens under the direct influence of mycorrhizal fungi and root tissues become more
177
resistant to pathogenic attack. Since the first
report of mycorrhiza-related chitinase in
tobacco (Dumas-Gaudot et al., 1992), additional ones have been demonstrated in various plant species. Lambias and Mehdy (1996)
evaluated the expression of mycorrhizaspecific chitinases and ß-1,3-glucanases in
soybean root infected with G. intraradices.
The efficacy of six AM species, A. morrawae, G. margarita, G. fasciculatum, G.
macrocarpum, S. calospora and Sclerocystis rubiformis obtained from rhizosphere of
C. microphyllus was evaluated for enhancement of PRO (peroxidase), PPO (polyphenol
oxidase) effect, with S. calospora being the
most promising of all the fungi. Good results
were observed with G. fasciculatum in
W. somnifera.
AM fungi ensure protection against certain soilborne pathogens (Diop, 1996). An
AM fungus influences microbial populations
and improves soil texture by the secretion of
mucilaginous compounds (Strullu et al.,
1991). Vesicles are lipid-filled and are initiated after the formation of arbuscules, but
live longer after the senescence of arbuscules
(Diouf et al., 2003). In Medicago truncatula,
at an early stage of arbuscule development
by G. versiforme, bright diffuse florescence
is seen around the arbuscular branches following antitubulin labelling. At later stages
of development, short microtubules are
closely associated with plasma membrane
surrounding the labyrinthine surface of the
arbuscule. γ Tubulin has been shown to be
associated with the nuclear envelope and
perifungal membrane in tobacco arbuscular
mycorrhizas (Genre and Bonfante, 1999).
Mycorrhizosphere changes in populations
of antagonists to specific pathogens depend
on having those antagonists present in background soil. If antagonists are absent and
deleterious microbes are present in significant numbers and enhanced by AM, the
incidence of disease can be increased
(Sharma et al., 2002).
Conclusions
The use of AM fungi as a biofertilizer is the
only alternative for successful farming.
178
A. Arya et al.
With this approach, we can obtain maximum return, not only for a season or a
year, but also over centuries. Mycorrhizal–
disease interaction has been studied in different plants by various workers. In many
cases, a better growth of plant is reported
after inoculations of an exotic AM fungi. A
low level of fungal aggressiveness and a
weak plant reaction are no doubt two key
factors that help in the establishment of a
successful symbiotic relationship between
the two organisms.
Increase in peroxide activity which is
localized in plant vacuoles and the cell
wall (Schloss et al., 1987) in AM-infected
root is one example of a mechanism of
plant resistance to microorganisms. Likewise,
production of phytoalexins (glyceolin I,
daidzein and coumestrol), chitinase, ß-1,3glucanases and b1 (PR) protein are important
components of AM-induced changes leading
to the resistance of the host to other pathogens. The systemic effect of AM fungi to Phytophthora infection in tomato has been
demonstrated by Pozo et al. (2002). Spores of
AM fungi are reported from different soils in
the country. These symbionts improve plant
growth. Their utilization as biocontrol agents
is gaining importance after many successful
trials. Multiplication and inoculum production of indigenous efficient AM fungi should
be undertaken. Efforts are needed to commercialize these novel microbes to bring about a
second green revolution in the country.
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15
Role of Fungal Endophytes
in Plant Protection
S.K. Gond, V.C. Verma, A. Mishra, A. Kumar and R.N. Kharwar
Mycopathology and Microbial Technology Laboratory, Centre of Advanced Study
in Botany, Banaras Hindu University, Varanasi, India
Abstract
Endophytes are the microorganisms that reside inside healthy plant tissues without causing any
detectable disease symptoms to the host. Often, each and every plant harbours either one or a battery
of endophytic microorganisms. The study of endophytes is now on a voyage of interest, not only
because of their role in filling the divide between discovered and undiscovered microbial diversity,
but also due to their harbouring a great potential to produce novel natural products. Other than soil,
higher plants also act as an alternative resource to isolate potential microorganisms. Natural compounds ranging from crop protection to human welfare have been isolated from this alternative source
of endophytes. Several anticancer, antibiotic, antimycotic, antiviral, antioxidant, nematicide, insecticide and immunosuppressive compounds have been reported from endophytes, such as cytochalasines, ambuic acid, oocydin, jesterone, cryptocandin, lolitrem B, and 3-hydroxypropionic acid and
taxol, etc. Many of them produce some toxic alkaloids and protect their hosts from herbivores. They
also improve the growth and yield of crops under various stressed conditions. Endophytic fungi have
been emerging as a new tool in genetic engineering, the pharmaceutical industry and in crop protection as well. In this chapter, the ability and role of endophytic fungi to ward off pests and environmental stresses on plants is discussed.
Introduction
The use of agrochemicals as a single control
measure in the field to protect crops from
their pests has been generating resistance in
these pests, and also represents a high risk
to field workers and consumers. Most of
these chemicals are non-biodegradable and
are responsible for polluting the environment. The control of phytopathogens has
relied mostly on chemical control agents
such as methyl bromide (Jarvis, 1993) but
after the Montreal Protocol (1991), the manufacturing of and trade in methyl bromide
was phased out in 2005. The utilization of
biological materials is an alternative and
safe way to protect plants from phytopathogens. The control of plant pathogens by
phylogenetically diverse microorganisms
acting as natural antagonists has been demonstrated repeatedly over the past 100 years.
The antifungal ability of Trichoderma sp.
has been well known since the 1930s and
extensive efforts have been made since then
to use them seriously for plant disease control (Harman, 1996).
Although the term ‘endophyte’ was used
much earlier in 1866 by German scientist,
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
183
184
S.K. Gond et al.
Heinrich Anton De Bary, the presence of
endophytes in plant was only recorded in
1904 when Freeman (1904) described an
entire plant’s endophyte life history in the
seeds of darnel (Lolium perenne sub sp.
temulentum = L. temulentum). This is a
topographical term and includes bacteria,
fungi, actinomycetes and algae which spend
their whole life, or a period of their life
cycle, inside healthy plant tissue without
causing any disease symptoms. Among all
endophytes, after bacteria, fungi are dominant in higher plants (Figs. 15.1 and 15.2). It
seems that other microbial forms almost
certainly exist in plants as endophytes such
as mycoplasmas (pleuro pneumonia-like
organisms – PPLO), rickettsia and archaebacteria; however, no evidence of them has
yet been observed. On the basis of their nature,
endophytes may be categorized in three
groups: (i) pathogens of another host that are
non-pathogenic in their endophytic relationship; (ii) non-pathogenic microbes; and
(iii) pathogens that have been rendered nonpathogenic but still capable of colonization
by selection methods or genetic alteration
Fig. 15.1.
(Backman and Sikora, 2008). Endophytes are
recorded from lower plant to higher plant
hosts (Stone et al., 2000). Each and every
plant is a reservoir of one or a suite of endophytes. In angiosperms, Poaceae members
are studied more for their endophytes.
Endophytic fungi are now attracting
great interest from researchers as an alternative source in controlling plant and human
pathogens. Some of the earlier workers before
the 1970s documented the endophytic fungi
residing inside the plant, exploring the biodiversity of hidden fungi. The period of 1981
to 1985 can be considered a historical one in
the study of endophytes, as plant protection
against herbivore insects was demonstrated
by endophytic microorganisms. Webber
(1981) demonstrated for the first time the
role of endophytic Phomopsis oblonga in the
protection of elm trees against the beetle
Physocnemum brevilineum. This report generated interest in the role of endophytes in
plant protection. Now, their beneficial role
to plants as well as to humans is being considered. In this regard, a large number of
antimicrobial compounds have been isolated
Endophytic fungal mycelia and spores within plant tissue stained with aniline-blue.
Role of Fungal Endophytes
Fig. 15.2.
185
Leaf pieces in Petri plate (21 days old) showing emergence of endophytic fungal mycelia.
from these endophytic microorganisms (Strobel, 2002, 2003; Zhang et al., 2006; Kharwar
et al., 2009). Endophytic fungi are now recognized as a new tool in the production of
antimicrobials and pharmaceutical compounds. In the search for bioactive compounds, several endophytic fungi have been
reported from the medicinal plants of North
India (Gond et al., 2007; Verma et al., 2007;
Kharwar et al., 2008).
Antimicrobials and their Activities
Produced from Endophytes
Antifungal activity of endophytes
Fungi are major causal organisms of various
diseases in plants. Many synthetic fungicides are available on the market, but they
are giving resistance to pathogens and are
also assisting in increasing the hazards to
human health. Data show that 52.3% of
endophytic fungal fermentation broths
display growth inhibition to at least one
pathogenic fungus, such as Neurospora sp.,
Trichoderma sp. and Fusarium sp. (Huang
et al., 2001). In a similar study, fermentation
broths of 9 (4.8%) out of 187 endophytic
fungi isolated from mainly woody plants
were highly active against Phytophthora
infestans in tomato plants (Park et al., 2005).
Induced resistance against Fusarium wilt by
endophytic F. oxysporum was generated in
tomato plants (Duijff et al., 1998). Sclerotinia
sclerotiorum is a common root, crown and
stem rot causing pathogen to several hosts
such as cabbage, common bean, citrus, celery,
coriander, melon, squash, soybean, tomato,
lettuce, cucumber, etc. Cyclosporine is characterized as a major antifungal substance
against S. sclerotiorum from the fermentation
broth of endophytic F. oxysporum (Rodriguez et al., 2006). Out of 510 isolates of
endophytic fungi, 64 isolates gave antifungal activities against Candida albicans, C.
glabrata, C. krusei, Cryptococcus neoformans,
Aspergillus fumigatus, A. flavus, Rhizopus
oryzae, Trichophyton rubrum and Microsporum canis (Anke et al., 2003).
Narisawa et al. (2000) found that the
root endophytic hyphomycete, Heteroconium
chaetospira, suppressed Verticillium sp. in
186
S.K. Gond et al.
Chinese cabbage in the field. Verticillium
wilt is one of the most destructive diseases
of aubergine. Eleven out of 123 isolates of
endophytic fungi, especially H. chaetospira,
Phialocephala fortinii, Fusarium, Penicillium, Trichoderma and Mycelium radicis
atrovirens ( MRA), after being inoculated on
to axenically reared aubergine seedlings,
almost completely suppressed the pathogenic effects of a post-inoculated, virulent
strain of V. dahliae (Narisawa et al., 2002).
Out of 39 endophytes of Artemisia annua,
21 showed in vitro antifungal activity
against a number of fungal pathogens (Liu
et al., 2001). The extracts of endophytic
Alternaria sp., isolated from medicinal
plants of the Western Ghats of India, inhibited the growth of C. albicans (Raviraja
et al., 2006). Colletotrichum gloeosporioides
was isolated as an endophyte from healthy
leaves of Cryptocarya mandioccana, giving
antifungal activity against phytopathogenic
fungi Cladosporium cladosporioides and C.
sphaerospermum (Inacio et al., 2006). Fungal endophytes Chaetomium and Phoma sp.,
isolated from asymptomatic leaf of wheat,
reduced the number and the area of pustules
of Puccinia recondita f. sp. tritici. A study
showed 40%, 65% and 27% antagonistic
interaction by endophytic morphospecies
in vitro against cacao pathogens, Moniliophthora roreri, P. palmivora and Crinipellis
perniciosa, respectively, while in the field
the endophytic C. gloeosporioides produced
a significant decrease in pod loss (Mejia
et al., 2008).
Cryptosporiopsis quercina is an endophytic fungus of a medicinal plant, Tripterigium wilfordii. It was observed that C.
quercina produced an antimycotic compound, cryptocandin, which was active
against a number of human and plant pathogenic fungi, including C. albicans, S. sclerotiorum and Botrytis cinerea (Strobel et al.,
1999a). A number of antifungal compounds
have been identified by Yue et al. (2000)
from the cultures of Epichloe and Neotyphodium species which showed activity
against chestnut blight fungus, Cryphonectria parasitica. These compounds were
indole derivatives, indole-3-acetic acid and
indole-3-ethanol, a sesquiterpene and a
diacetamide. Consequently, a tetramic acid,
cryptocin, has also been isolated from the
cultures of C. quercina, which exhibits strong
antifungal activity against Pyricularia oryzae,
the causal agent of blast of rice, as well as
some other plant pathogenic fungi (Li et al.,
2000).
Colletotrichum gloeosporioides was isolated from A. mongolica, which produced
antifungal metabolite colletotric acid, against
the fungus Helminthosporium sativum (Zou
et al., 2000). Another Colletotrichum sp.,
isolated from A. annua, produced bioactive
metabolites that were fungistatic to several
plant-pathogenic fungi (Lu et al., 2000).
Pestalotiopsis microspora is a commonly
isolated and well-identified fungus from
every rainforest plant and, as a single endophytic species, it contributes a high percentage to the total mass of fungal endophytes in
any host. Pestalotiopsis is observed to produce many antimicrobial secondary metabolites. One such secondary metabolite is
ambuic acid, an antifungal agent which has
been described from several isolates of P.
microspora (Li et al., 2001). P. jesteri, isolated from the Sepik River area of Papua New
Guinea, produced jesterone and hydroxyjesterone which exhibited antifungal activity
against a variety of plant-pathogenic fungi
(Li and Strobel, 2001). Two new metabolites,
ethyl 2,4-dihydroxy-5,6-dimethylbenzoate
and phomopsilactone, have been isolated
from P. cassiae, an endophytic fungus in
Cassia spectabilis, with strong antifungal
activity against the phytopatogenic fungi, C.
cladosporioides and C. sphaerospermum
(Silva et al., 2005).
An aquatic plant, Rhyncholacis penicillata, is known worldwide to harbour a
potent antifungal microbe, Serratia marcescens, which produces an antioomycetous
compound named oocydin A (Strobel et al.,
1999b). Oocydin A provides the plants with
a strong protection against several water
moulds.
Antibacterial activity of endophytes
The antimicrobial activity of endophytic fungi
has been observed in a range of bacteria
Role of Fungal Endophytes
representing pathogens to plants and humans.
The broths of 16 endophytic fungi isolated
from the medicinal herb, Cynodon dactylon
(Poaceae), were identified as having potent
anti-Helicobacter pylori activity. The most
active endophyte, identified as Aspergillus
sp. (strain number: CY725), produced four
active fractions and was identified as: (i)
helvolic acid; (ii) monomethylsulochrin;
(iii) ergosterol; and (iv) 3β-hydroxy-5α,
8α-epidioxy-ergosta-6, 22-diene with corresponding MICs of 8.0, 10.0, 20.0 and 30.0 µg/
ml against H. pylori, respectively (Li et al.,
2005). Bioactivity of endophytic fungi of Coffea arabica and C. robusta was screened
against Salmonella choleraesuis, Staphylococcus aureus, Pseudomonas aeruginosa and
four different Escherichia spp. Out of these
endophytic fungi, T. harzianum, Guignardia
sp. and Phomopsis sp. have inhibited four to
five bacterial species successfully (Sette et al.,
2006). Out of 377 isolates of endophytic fungi
from Garcinia plants, 18.6% isolates displayed antimicrobial activity against at least one
pathogenic microorganism, such as S. aureus,
a clinical isolate of methicillin-resistant
S. aureus, C. albicans and C. neoformans
(Phongpaichit et al., 2006).
Epicoccum purpurascens and Truncatella hartigii were found to have significant
action against human pathogenic bacteria.
E. purpurascens expressed a good antibacterial effect on S. aureus and P. aeruginosa
and a very good antibacterial effect on E.
coli, while T. hartigii exhibited a significant
antibacterial effect on Enterococcus faecalis
(Janes et al., 2007). Fusarium was the most
frequently isolated endophyte from the Chinese traditional medicinal plant, Dioscorea
zingiberensis, and F. redolens showed the
most potent antibacterial activities against
B. subtilis, S. haemolyticus, E. coli and X.
vesicatoria (Xu et al., 2008).
Two antibacterial cerebrosides, one
new and another known, were isolated from
Fusarium sp., an endophytic fungus found
in Quercus variabilis. The new cerebroside
was named fusaruside with structure (2S,2′R,
3R,3′E,4E,8E,10E)-1-O-b-d-glucopyranosyl2-N-(2′-hydroxy-3′-octadecenoyl)-3-hydroxy9-methyl-4,8,10-sphingatrienine. Both of
them were active against B. subtilis, E. coli
187
and P. fluorescens (Shu et al., 2004). Periconicins A and B were isolated from
endophytic fungus Periconia sp. of Taxus
cuspidata and exhibited antibacterial activity against many pathogenic bacteria. The
minimum inhibitory concentration (MIC) of
periconicin A was even less (3.12 µg/ml)
than that of gentamicin (12.5 µg/ml) against
Klebsiella pneumoniae (Kim et al., 2004).
The endophytic fungus, Xylaria sp., isolated
from Ginkgo biloba, showed strong antibacterial activity in vitro against S. aureus
(MIC 16 µg/ml), E. coli (MIC 10 µg/ml), S.
typhae (MIC 20 µg/ml) and S. typhimurium
(Liu et al., 2008). Recently, some bioactive
nitronaphthalenes have been isolated from
endophytic fungus, Coniothyrium sp. (Krohn
et al., 2008). Javanicin, an antibacterial
naphthaquinone, has been isolated from
neem endophyte, Chloridium sp., which
was significantly active against Pseudomonas spp. (Kharwar et al., 2009).
Antiviral activity of endophytes
Viruses are an important causal agent of various diseases in plants and animals. Endophytes can induce plant resistance against
viral diseases, but there is a contradiction
and Guy (1992) found no correlation between
virus infection and the incidence of endophyte in perennial ryegrass (L. perenne),
whereas other correlative studies have
revealed that some endophyte-infected tall
fescue (Festuca arundinaceum) seem to be
more resistant to barley yellow dwarf virus
(BYDV) than the others (Mahmood et al.,
1993; Guy and Davis, 2002). Lehtonen et al.
(2006), when releasing the viruliferous
aphid vectors to endophyte-infected and
endophyte-free L. pretense plants in a common garden, found the number of aphids
and the percentage of BYDV infections were
lower in endophyte-infected plants compared to endophyte-free plants. Human
cytomegalovirus (hCMV) is a ubiquitous
opportunistic pathogen. Two novel human
cytomegalovirus protease inhibitors, cytonic
acids A and B, have been isolated from the
solid-state fermentation of the endophytic
fungus, Cytonaema sp. (Guo et al., 2000).
188
S.K. Gond et al.
Nematicidal activity of endophytes
Insecticidal activity of endophytes
Endophytic fungi are known to produce
some compounds which are toxic to nematodes. The first report on antagonistic activity of endophytic fungi against plant
parasitic nematodes was observed in tall
fescue (F. arundinacea) infected by Pratylenchus scribneri. The nematode population was found to be comparatively less in
the soil surrounding endophyte-infected
plants. Since the root of tall fescue (F. arundinacea) was infected by Acremonium
coenophialium, it was considered that the
presence of A. coenophialium deterred the
nematode population. The colonization of
fungal endophyte, F. oxysporum, in the
roots of tomato plant reduced 60% infection
of Meloidogyne incognita successfully.
Endophyte-free perennial ryegrass plants are
shown to have a larger number of M. incognita population in roots than endophytecontaining plants (Ball et al., 1997).
Pregaliellalactone and structurally related
lactones were isolated with nematicidal
activity from non-graminaceous endophytes
and related saprophytic ascomycetes (Kopcke et al., 2002a,b). Another endophytic
microbe, Burkholderia ambifaria, isolated
from corn root, produced some toxic metabolites which inhibited egg hatching and
mobility of second-stage juveniles of M.
incognita (Li et al., 2002).
Diedhiou et al. (2003) demonstrated the
successful nematicidal activity of an arbuscular mycorrhiza, Glomus coronatum, and an
endophytic fungus, F. oxysporum, against
the M. incognita in tomato plant. Several
endophytic fungi isolated from above-ground
plant organs produced 3-hydroxypropionic
acid (HPA) by bioactivity-guided fractionation of extracts and showed selective nematicidal activity against the plant-parasitic
nematode, M. incognita, with LD50 values of
12.5–15 µg/ml (Schwarz et al., 2004). Radopholus similis is an important parasitic
nematode on banana and other plants. It is
suggested that the dual inoculations of
endophytic fungal isolates reduce a large
number of the R. similis population (Felde
et al., 2006).
Fungi are known to produce a large number
of insecticidal metabolites such as destruxins, ibotenic acid, pantherine, tricholomic
acid, etc. Endophytic fungi are also known
to deter insect pests (Clay, 1989; Carroll,
1991, 1995; Azevedo et al., 2000). Several
toxins are produced by endophytic fungi
and these substances confer host protection
against different herbivores. The endophytic
fungus, P. oblonga, was responsible for
reducing the spread of Dutch elm disease
causal agent, Ceratocystis ulmi, by controlling
its vector beetle (P. brevilineum) (Webber,
1981). In 1985, Claydon and his co-workers
confirmed that endophytic fungi belonging
to the Xylariaceae family synthesized secondary metabolites in host Fagus sp. and
that these substances affected the beetle larvae. Susceptible and resistant cultivars of
perennial rye grass (L. perenne L.) against
sod webworms (Crambus spp.) were analysed for the presence of an endophytic fungus. All resistant cultivars were found to
have a high infection of endophytic fungi.
Several highly infected ryegrass species
with endophytic fungi consequently have
shown less attack frequency of Argentine
stem weevils (Listronotus bonariensis)
(Gaynor and Hunt, 1983). Barker et al. (1984)
and Prestidge et al. (1984) also observed
that the same grass infected with endophytic Acremonium sp. was more resistant
to stem weevils in New Zealand. In the
white spruce, Picea glauca, the death rate of
the Homoptera, Adelges abietis, was considerably higher when galls were infected
with the endophytic fungus, C. sphaerosperum (Lasota et al., 1983). In L. perenne and
a few members of genus Cyperus, insectpest Spodoptera frugiperda was affected
adversely by endophytic fungus like Balansia cyperi (Clay et al., 1985a,b). Ahmad
et al. (1985) showed that endophytic Acremonium sp. deterred the grasshopper,
Acheta domesticus. Patterson et al. (1992)
observed the production of alkaloids by
endophytic Acremonium in plants Lolium
and Festuca that reduced the attack of the
Japanese beetle, Popilla japonica. Muscodor
Role of Fungal Endophytes
vitigenus, an endophytic fungus of Paullinia
paullinioides, from the Peruvian Amazon,
is known to produce naphthalene, which
effectively repels the adult stage of the wheat
stem sawfly, Cephus cinctus (Daisy et al.,
2002). Endophyte-mediated resistance was
reported in strong creeping and chewings
fescue species against red thread (Bonos
et al., 2005). Beauveria bassiana is a highly
effective entomopathogen of a wide range of
insects. Grass varieties infected by Neotyphodium endophyte have affected the feeding performance and preference of newly
hatched nymphs of the hairy chinch bug,
Blissus leucopterus hirtus, a common turfgrass pest in north-eastern USA (Steeve et al.,
2007). Akello et al. (2007) incorpotated B.
bassiana as an artificial endophyte in banana
plants to combat the banana weevil, Cosmopolites sordidus. The endophytic fungi,
B. bassiana and Clonostachys rosea, isolated
from coffee plant, showed strong antagonistic activity against coffee berry borers (Vega
et al., 2008).
Two new insecticidal compounds,
5-hydroxy-2-(1′-oxo-5′-methyl-4′-hexenyl)
benzofuran and 5-hydroxy-2-(1′-hydroxy5′-methyl-4′-hexenyl) benzofuran were isolated via bioassay-directed fractionation of
culture extracts of an unidentified endophytic
fungus obtained from wintergreen, Gaultheria
procumbens (Findlay et al., 1997). These
compounds exhibited toxicity to spruce
budworm (Choristoneura fumiferana Clem.)
cells. Peramine and lolines, potent insecticides, are produced in endophyte-infected
perennial ryegrass and protect them from the
Argentine stem weevil, Listronotus bonariensis (Rowan and Latch, 1994; Tanaka et al.,
2005). Nodulisporic acids, novel indole
diterpenes, have potent insecticidal properties against the larvae of the blowfly by
activating insect glutamate-gated chloride
channels. Nodulisporium, an endophytic
species from the plant, Bontia daphnoides,
produces such nodulisporic compounds
(Demain, 2000). A strain of endophytic Penicillium sp., isolated from the fresh roots of
Derris elliptica, produces some insecticidal
compound analogues to rotenone against
the adult turnip aphid, Lipaphis erysimi
(Hu et al., 2005).
189
Plant Protection in Abiotic Stresses
Endophytes are also involved in the protection of plants in various abiotic stresses like
drought, temperature, pH, heavy metals, etc.
(Rodriguez et al., 2004). Water stress tolerance was observed in epacrids and their
endophytic partners in south-west Australia
(Hutton et al., 1996). In drought conditions,
water content of some endophyte-associated,
field-grown tall fescues may be maintained
at higher levels than those of endophyte-free
plants (Elbersen and West, 1996; Buck et al.,
1997). This phenomenon may be explained
by enhanced accumulation of solutes in tissues of endophyte-infected plants as compared to non-infected plants, or by reduced
leaf conductance and a slowdown of the
transpiration stream, or due to thicker cuticle
formation (Malinowski and Belesky, 2000).
The endophytic mutants and wild-type
C. magna confer drought tolerance that
allows symbiotic tomato and pepper plants
to survive desiccation for 24 and 48 h longer than non-symbiotic plants, respectively
(Redman et al., 2001). Endophytic colonization was observed to increase the minimum
leaf conductance in Theobroma cacao, a
measure of leaf water loss after maximal stomatal closure under drought stress (Arnold
and Engelbrecht, 2007). However, no evidence for endophyte-mediated drought tolerance was observed in Acremonium-infected
tall fescue (White et al., 1992). It is suggested that endophyte-mediated drought
resistance may be due to alterations in
drought avoidance.
Malinowski and Belesky (1999) observed
that the pH of a limed, acidic soil increased
faster as a result of the root activity of
endophyte-infected tall fescue compared
with non-infected plants under phosphatedeficient conditions. Liu et al. (1996) observed
that aluminium tolerance in endophyteinfected fine fescues (Festuca spp.) was
greater as compared to non-infected plants.
In an experiment, endophyte-infected clone
grew significantly better in high aluminium soils relative to the endophyte-free
clone (Zaurov et al., 2001). L. perenne, symbiotic with N. lolii, showed higher values of
total dry weight and tiller number compared
190
S.K. Gond et al.
to non-symbiotic plants in Zn stress (Monnet
et al., 2001). In low NaCl salt stress condition,
the endophyte Piriformospora indica-infected
barley plants showed higher biomass than
non-infected plants (Waller et al., 2005). The
mechanism of endophyte-conferred salt tolerance has not been investigated so far.
In the USA, a plant species, Dichanthelium lanuginosum, has been found growing
in the geothermal soils of Yellowstone
National Park (YNP) and Lassen Volcanic
National Parks (LVNP) at temperatures as
high as 57°C (Stout and Al-Niemi, 2002).
Redman et al. (2002) observed that those
plants colonized by an endophytic fungus,
Curvularia protuberata, were able to tolerate the higher temperature and, thus, we
might conclude that endophytes supported
the plant to withstand heat or drought
stresses. However, an in-depth investigation by Marquez and his colleagues (2007)
showed that this was not only because of
plant–fungus symbiosis but it also included
a virus as a third partner, which parasitized
on C. protuberata. Thus, it is a complex tripartite symbiosis and the heat tolerance
ability of the fungus is, in fact, related to the
virus. That mycovirus is called a Curvularia
thermal tolerance virus (CThTV).
Two mechanisms are involved in the
endophyte-conferred biotic and abiotic
stress tolerance: (i) rapid activation of host
stress response systems in exposure to stress
(Redman et al., 1999); and (ii) synthesis of
anti-stress biochemicals in the host, either
by endophytes or through endophyte induction (Bacon and Hill, 1996). Endophyteproduced anti-stress biochemicals are
mostly alkaloids. In addition to anti-stress
biochemicals, plant and fungal mutualism
has been maintained over an evolutionary
time by the ability of fungi to control the
activation of host stress response systems
and, in core, act as ‘biological triggers’
(Rodriguez et al., 2004). When a plant interacts with environmental biotic and abiotic
stresses, it produces several damaging reactive oxygen species (ROS). Therefore, it is
hypothesized that endophytes inside plants
scavenge these ROS rapidly and protect
their host (Rodriguez and Redman, 2005;
Tanaka et al., 2006).
Indian Contributions to
Fungal Endophyte Research
The past history of endophytic research in
India, especially with fungi, is not so encouraging. It seems that workers who started this
research in India are still actively involved
in advancing their research manifesto with
this ‘under-studied’ group of microbial
population and have not advanced to the
fields and forests of the countryside looking
for novel microbe/plant associations. Prof
Suryanarayanan and his group (Chennai)
have initiated biodiversity and distribution
patterns of fungal endophytes with some
medicinal plants in India and have published several papers along this line. He has
also isolated some bioactive compounds
and melanin from endophytic fungi (Suryanarayanan et al., 2004). Several research
groups have started paying more attention
to various aspects of endophytic fungi.
No more than a dozen research groups at
various locations in India are vigorously
involved in either biodiversity or natural
product discovery from this untapped and
alternative resource (Table 15.1).
It has become obvious to many workers
throughout the world that endophytic
microbes have enormous potential to solve
many of mankind’s problems. Thus, with
the discovery of new compounds, we can
protect our agriculture and medicine industries, as well as plant health. After more than
20 years of effort, the total number of publications from Indian researchers, including
some fairly recent ones (Shankar et al.,
2003; Seena and Sridhar, 2004; Amna et al.,
2006; Tejesvi et al., 2007; Gangadevi and
Muthumarry, 2008), is relatively small.
Due to the great variation in plant biodiversity and seasonal changes in India, we
may have a better opportunity to collect/
isolate various types of promising endophytic fungi, especially from rainforests and
mangrove swamps, which may be able to
produce an enormous variety of potential
bioactive natural compounds. An increasing
population of AIDS and immunocompromised patients in India compels us to bear
them in mind when searching for safe drugs.
Table 15.1. List of Indian workers involved in endophytic fungal research.
Name of group leader
Place of work
Work specialization
E-mail addresses
1.
Dr T.S. Suryanarayanan
2.
Dr K.R. Sridhar
Dr H.S. Prakash
4.
5.
Dr D.J. Bhat
Dr J. Muthumarry
6.
7.
Dr Arun Arya
Dr R. Uma Shaankar
8.
Dr Absar Ahmad
National Chemical Laboratory, Pune
9.
10.
Dr S.K. Singh
Dr S.C. Puri/
R.K. Khajuria
Dr R.N. Kharwar
Agharkar Research Institute, Pune
Regional Research Laboratory, Kanal Road,
Jammu Tawi
Department of Botany, B.H.U., Varanasi-221005
Endophytic fungal diversity and
natural product discovery
Endophytic fungal diversity and
bioactive molecules
Endophytic fungal diversity and
bioactive molecules
Endophytic fungal diversity
Endophytic fungal diversity and
bioactive fungal compounds
Endophytic fungal diversity
Endophytic fungal diversity and
bioactive molecules
Synthesis of silver and gold particles
from endophytes
Endophyte diversity
Natural product development
tssury@md3.vsnl.net.in
ts_sury2002@yahoo.com
sirikr@yahoo.com
3.
Department of Botany, Vivekanand College,
Chennai
Department of Biosciences, Mangalore University,
Mangalagangotri, Mangalore
Department of Botany, University of Mysore,
Manasagangotri
Department of Botany, Goa University, Panji, Goa
Department of Botany, University of Madras,
Chennai
Department of Botany, MSU, Baroda
University of Agriculture, Karnataka
singhsksingh@rediffmail.com
khajuriark@yahoo.com
Endophytic fungal diversity and
natural product discovery
rnkharwar@yahoo.com
kharwar1@rediffmail.com
11.
hasriprakash@gmail.com
bhatdj@rediffmail.com
mm_j@rediffmail.com
aryaarunarya@rediffmail.com
umashaanker@gmail.com
aahmad@dalton.ncl.res.in
Role of Fungal Endophytes
Sr. No.
191
192
S.K. Gond et al.
India really needs a variety of novel antimicrobial compounds of biological origin, so
that we can solve the problems of ecofriendly farmers and the weaker sections of
society in which the above-mentioned diseases are prevalent. The fungi, as a group,
hold enormous potential as sources of antimicrobials. Observations prove that this
group of organisms resides inside healthy
plant tissues as endophytes without causing
any detectable symptoms. Therefore, we
feel strongly that India needs to gear up and
exact its research to exploit the maximum
potential of the promising endophytes for
natural product discovery, which could at
least facilitate some of the existing problems
of its huge population.
Conclusions
In the study of mycodiversity, we often forget the endospheric fungi as researchers
focus their attention on the phyllospheric
and rhizospheric fungi. The endosphere is a
special niche where endophytic microorganisms reside and, in response, produce a
variety of metabolites, which are mostly
toxic to plant and human pathogens. In this
aspect, plant pathogens interact with the
plant itself, as well as the plant’s endophytes.
The role of endospheric or so-called endophytic fungi in plant protection is quite clear
in the above-mentioned examples. Besides
protecting plants from biotic and abiotic
stresses, endophytes also improve the health
and yield of plants by producing some
growth-regulating phytohormones. Although
endophytes are still poorly investigated
microorganisms, they have shown that they
are going to play a prominent part in the discovery of many bioactive natural compounds.
Bioactive natural products of endophytic origin can change the scenario of existing agropesticides because of their easy and sustainable
production. Many scientists throughout the
world are engaged in the search for bioactive compounds from endophytes. There is
a gap in the knowledge on the genetic and
biochemical communications between the
plant and endophytic symbionts. We have
to minimize this gap for better utilization of
endophytic microorganisms.
Acknowledgements
The authors are thankful to the Head of the
Department of Botany, BHU, Varanasi, for
providing the necessary facilities. They also
extend their thanks to the CSIR, New Delhi,
for financial support.
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Zhang, H.W., Song, Y.C. and Tan, R.X. (2006) Biology and chemistry of endophytes. Natural Product
Reports 23, 753–771.
Zou, W.X., Meng, J.C., Lu, H., Chen, G.X., Shi, G.X., Zhang, T.Y. and Tan, R.X. (2000) Metabolites of
Colletotrichum gloeosporioides, an endophytic fungus in Artemisia mongolica. Journal of Natural
Products 63, 1529–1530.
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Part V
Managing Fungal Pathogens
Causing Leaf Damage
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16
The Rust Fungi: Systematics,
Diseases and Their Management
M.S. Patil1 and Anjali Patil2
1Department
of Botany, Shivaji University, Kolhapur, India; 2Department of Botany,
Rajaram College, Kolhapur, India
Abstract
The rust fungi (Uredinales) consist of 7000 species belonging to 163 genera in 14 families and comprise about 10% of all described species in the Kingdom Fungi. All the rust fungi are ecologically
obligate parasites on ferns, gymnosperms and angiosperms. There are six vital processes in plants and,
correspondingly, six ways in which rusts affect their hosts adversely. Rusts as pathogens damage foliage, the main organ of photosynthesis, destroy seedlings, impair growth and interfere in the metabolism of the hosts. Management of any disease begins with correct identification of the pathogen; hence,
some important concepts in rust systematics are discussed, along with detailed information about rust
diseases of some economically important crops. Of course, discussion on plant diseases would not be
complete without recent management strategies. The discussion includes the following;
1. Rust systematics, including characteristic features of rust fungi, their occurrence and geographical distribution, vegetative and reproductive propagules, pleomorphism, autoecious and heteroecious
nature and host range, etc.
2. Rust diseases of crops, including field crops – medicinal, ornamental, cereals, pulses, millets,
oilseeds, fruit and plantation crops, etc. – nature of disease, epiphytotics, disease development index;
X = XoeRT, assessment of crop losses.
3. Management strategies citing food crisis, need for another green revolution, crop losses, famines,
social impact of rust diseases, e.g. change in coffee-drinking habit due to coffee rust, management
methods – Sharvelle’s strategy (1961): (i) protective, (ii) preventive; and (iii) corrective (physiological disorders), cultural, chemical, biological, breeding, biotechnology – transgenic plants are described in detail.
Introduction: Rust Systematics
The rust fungi (Uredinales) consist of 7000
species belonging to 163 genera in 14 families and comprise c.10% of all described
species in the Kingdom Fungi (Kirk et al.,
2001; Ono, 2002). The Uredinales are
believed to be monophyletic taxa and recent
molecular–phylogenetic analysis (Swann and
Taylor, 2001) supports this perspective. Very
recently, a rust, Uredo vetus Henne, has
been reported for the first time on Selaginella sp. The parasitism of rust fungi to the
host plant is highly specific; however, this
specialization varies with species, for example, two well-known rusts of soybean, namely
Phakopsora pachyrhizi Syd. and Syd. and
P. meibomiae Arthur (American rust) (Ono
et al., 1992), occur on a large number of species of family Leguminosae. The rust fungi
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
201
202
M.S. Patil and A. Patil
are unique in having complex life cycle patterns with elaborate spore forms. Accordingly, many types of life cycles are known
(Laundon, 1973) with modified spore types
with immense functional diversities; for
example, aecioid teleutospores in endoform
rusts like Endophyllum, Monosporidium
and Kulkerniella or uredinoid teleutospores
in Hemileia vastatrix Berk. & Br., which
Rajendren (1967) described as ‘Kamat Phenomenon’. The spermogonial–aecial host/s
and uredinial–telial host/s are closely associated eco-geographically. Heteroecious life
cycle is widespread in the uredinales. If all
spore forms are produced in unidirectional
order, the life cycle is said to be macrocyclic
(Puccinia graminis, P. helianthi, the heteroecious and autoecious rusts of wheat and
sunflower, respectively). There is a tendency
to omission of spore form in life cycles and
thus many different patterns exist, for example, demi-cyclic, in which the uredial stage is
absent. In rust fungi, a widespread assumption is that parasitism and host specialization are acquired at an early stage of rust
fungus evolution. Nine types with 11 variations are found in nuclear cycles associated
in metabasidium development of microcyclic rust fungi (where only a telial stage with
or without a spermogonial stage is formed
on a plant throughout the season). Thus,
rust species that produce only teleutospores
that germinate without dormancy to initiate
new generations repeatedly in a single growing season would be highly adaptive, e.g. P.
pampeana Speg. on chillies (Capsicum spp.),
P. alyxiae Arthur on Alyxia spp., P. xanthii
Schw., M. machili (Hennings) T. Sato, E. acacia Hodges and Gardner. Microcyclic species
exhibit two or more patterns of nuclear cycles
and different metabasidium development,
indicating that microcyclic lineages might
have evolved independently and repeatedly
from macrocyclic parental species.
many economically important pathogens
of vascular plants. Dietel (1900) divided
the order Uredinales into four families based
on sessile or pedicillate teleutospores as
follows:
Families of Rust Fungi
Based on Teleutospores
1. Puccinia Pers. ex Pers. (c.3000–4000
spp.).
2. Uredo Pers. ex Pers. (c.3000 spp.).
3. Uromyces (Link) Unger (c.600–700 spp.).
4. Ravenelia Berkeley (c.200 spp.).
Rust fungi comprise one of the largest and
best described groups of fungi and include
Pedicillate teleutospores -------Pucciniaceae
Sessile teleutospores
In a single layer --------Melampsoraceae
In 1–2 layers forming waxy crust ----------------------------------Coleosporiaceae
In chains ---------------------Cronartiaceae
Families
Pucciniastraceae (Arthur)
Gaumann
Coleosporiaceae Dietel
Cronartiaceae Dietel
Melampsoraceae Schroeter
Phakopsoraceae (Arthur)
Cummins & Hiratsuka
Mikronegeriaceae Cummins &
Hiratsuka
Chaconiaceae Cummins &
Hiratsuka
Uropyxidaceae (Arthur)
Cummins & Hiratsuka
Pileolariaceae (Arthur)
Cummins & Hiratsuka
Raveneliaceae (Arthur) Leppik
Phragmidiaceae Corda
Sphaerophragmidiaceae
Cummins & Hiratsuka
Pucciniaceae Chevalier
Puccinosiraceae (Dietel)
Cummins & Hiratsuka
Unassigned genera
Total
Genera
06
02
01
02
11
01
10
10
03
14
10
06
15
09
99
08
107
However, this dependence on teleutospore
morphology has brought many unrelated
genera into the same family. Some of the
larger genera are:
The Rust Fungi
5.
6.
7.
8.
Melampsora Castagne (c.100 spp.).
Hemileia Berk. & Br. (c.50 spp.).
Coleosporium Lev. (c.80 spp.).
Phragmidium Link (c.60 spp.).
In some cases, the original function has
been changed irrespective of its basic nature
or structure; for example, the species of
Endophyllum, Monosporidium or Kulkerniella produce aeciospores morphologically
in aecial cups, but they function like teleutospores producing promycelium bearing
basidiospores and are thus called aecioid
teleutospores. In H. vastatrix, urediniospores
occasionally function like teleutospores and
are called uredinoid teleutospores. Of course,
in the first example, no teleutospores are
produced, while in the second example,
teleutospores normally are produced in the
life cycle. In the course of the evolution of
rust fungi, there is a tendency to eliminate
spores, narrowing the host range and surviving in spite of unfavourable environmental conditions or non-availability of the
required host. There are numerous examples in which a rust survives or continues
its life cycle by producing one type of
spore, e.g. Aecidium, Uredo, Peridermium,
Caeoma, etc.
203
repeating like conidia, germinate very easily within 24 h at high temperatures and
relatively higher humidity, but their viability
is lost at very high temperatures. At lower
temperatures, spores remain viable for a long
time. They germinate mostly by germ tubes,
except in coffee rust, where they behave like
uredino teleutospores.
Urediniospores are most significant in
disease development and spread on epiphytotic scale (van der Plank, 1963, 1968).
Hence, this spore state is also considered as
the conidial state during sporulation and the
release of spores form spore clouds in the
air and serves as secondary and tertiary
inoculum within the crop in a favourable
season. Rust diseases, due to their repeating
nature, are known as compound interest diseases. Log e(x/1 – x) where x is a proportion
of infected susceptible tissue, if the pathogen is systemic. Urediniospores are light,
airborne and travel long distances at various
heights; for example, wheat rust urediniospores travel from Mexico to Canada; in
Indian wheat rust, the rust originates in
South India from the Nilgiri and Pulney
Hills and travel via Central India from the
plateau of Mahabaleshwar and Panchgani to
North India.
Spore morphology
In groundnut rust, the uredinospores are
one-celled, spherical to oval or angular,
stalked, mostly brown coloured, faint or
dark, thick-walled with spiny, verrucose,
or a modification of these two, rarely
smooth, bearing visible areas in the walls
through which germination takes place.
Tulasne and Tulasne (1847) observed for
the first time, pores/oscules varying from
2 to 20 in number. The germ pores may be
distributed equatorially, zonal or scattered.
Cummins (1936) also recognized their phylogenetic significance in the rust taxonomy.
There appears to be some correlation between
the arrangement of pores and the shape of
urediniospores; globoid spores usually
have scattered pores, while ellipsoid, oblong
or asymmetrical spores usually have zonnate pores. Urediniospores are bi-nucleate,
Rust physiology – urediniospore
germination
Spore liberation is active and the terminal
velocity of fungal spores in the air is 0.05–
2.5 cm/sec. In calm weather, only 0.05%
spores travel more than 100 m from their
source of origin. Spores of black stem rust
fall from a height of 1.6 km to the ground at
a speed of 12 mm/sec and travel from place
to place at 11–32 km/h (Gregory, 1973).
Urediniospores of different rust species have
a different period of viability as they are
affected by environmental factors such as
RH, light intensity, as well as their own
structural characteristics, namely wall
thickness, etc. In northern India, urediniospores are killed by high temperatures in
the field and cannot serve as a source of
inoculum the following year.
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M.S. Patil and A. Patil
Viability of urediniospores in rust fungi
Rust urediniospores show different periods
of viability at 20–40% RH and 23°C, e.g. P.
graminis tritici 36 days, P. recondita 63 days,
P. coronata 87 days, P. menthae 173 days, P.
helianthi 185 days and U. pisi 75 days. Viability of urediniospores decreases at higher
or lower humidity. Spores germinate generally at 90–100% RH, while the temperature
requirement varies greatly in different species. The period required for sporulation
(urediniospores) in black stem rust of
wheat is found to be 5 days at 24°C. If the
temperature is lowered to 0°C, then sporulation occurs after 85 days. Spore longevity
depends on light, temperature, relative humidity, species of rust and type of spore. Basidiospores and pycniospores are delicate
and have least viability. But if the spores are
kept at a low temperature, viability lasts for
18 days. In the case of sunflower rust, relative humidity is more important than temperature. At 80% RH, only 5% aeciospores
remain viable after 56 days.
Teleutospores spores are produced at
the end of a rust fungus’s life cycle, i.e. spores
terminating the life cycle of rusts. They are
produced in telia in or on the host, are innate
or erumpent, covered or exposed in telial
sori, in the leaves or in the stem. There is a
tendency in rusts to eliminate spore states
showing progressive reduction either due to
non-availability of host or climatic conditions, for example, rusts in temperate regions
on the family Liliaceae. Rust systematics, a
dynamic science, is far from perfect; hence,
there is still a lot of work to be done on their
taxonomy and pathology. Study of their
host’s behaviour during development, varieties, races, physiological forms and pathotypes is beyond the scope of taxonomists.
Rust Diseases of Some Economically
Important Crops
Plant pathology originated in Europe and
migrated to North America, where it flourished and spread to different parts of the
world. Among all known living organisms,
three groups are dominant, namely insects,
flowering plants and fungi. Among all these
plant pathogens, fungi are the most dominant and successful plant pathogens and are
estimated to produce more than 25,000 diseases. Among these, 8000 diseases of cultivated and plantation crops are extremely
damaging in the field every year. Kuhn,
(1858) wrote a book entitled Diseases of
Cultivated Plants. Rusts are complex; hence,
it is difficult to understand how they damage standing crops in the field qualitatively
and quantitatively, creating problems of
food crisis and insecurity; a global problem
today.
Epidemiological Studies
Epidemiology is the science of epidemics or
diseases in plant population. Types of epiphytotics are:
1. Based on the rate of disease development:
(i) Tardiv (slow epiphytotics);
(ii) Explosive (rapid epiphytotics).
2. Area covered and time of development:
(i) Pandemic – developing on a continental scale;
(ii) Sporadic – seasonal and irregular
incidence.
There are also secondary epiphytotics known.
Epiphytotics is also defined as ‘a host–
pathogen system, out of genetic balance in
favour of the pathogen’. Such epiphytotics
of crop plant diseases are known, in the history of plant pathology, to be followed by
food famines:
1. Wheat rust epidemics occurred in 1916
in America and Canada; 1935 and 1937 in
America; 1951 in Europe; and 1827, 1907,
1947, 1949–1950, 1957, 1971–1972 in India.
2. Coffee rust epidemics occurred in 1867
and 1875 in Sri Lanka; 1891 in the Philippines; 1891–1892 in Java; 1911–1913 in Central Africa; 1871–1878 in South Africa; and
1970–1971 in Brazil.
The coffee rust famines influenced the coffeedrinking habit, which then changed to tea.
The Rust Fungi
Rusts are compound interest diseases
and an increase of infection at a compound
interest rate exponentially/logarithmically
increases the rate of compound interest of
disease by primary and secondary infection.
The compound interest equation can be
given as:
X = XoeRT
where X = the amount of disease at time
T, Xo = initial amount of disease at O time,
R = infection rate, which is variable, and
e = 2.718 for cereal rusts.
The rate (R) of increase % per unit of
time is a fundamental concept in epidemiology, e.g. 12.5%/day in P. recondita and
57%/day in Phytophthora infestans. Development of epiphytotics is basically a transport problem to get enough inoculum to the
right place at the right time. Plant–pathogen–
environment is a triple interaction and may
be complicated by vectors and humans;
according to van der Plank (1963, 1968), the
pathogen must be virulent. To express virulence quantitatively, the disease reaction
type is expressed in numerical values as:
R (resistant) = 01, MR (moderately
resistant) = 02, S (susceptible) = 03
Aggressiveness corresponds to disease severity on a 0–9 score scale:
0 = absent, 9 = more than 75% leaf
area in 12 days after inoculation
therefore
VI (virulent index) = [1 + (virulence ×
aggressiveness) × latent period]
VI = [1 + VAL – 1]
where VI = virulent index, A = aggressiveness and L = latent period.
205
The estimated annual crop losses worldwide (Agrios, 2005) are:
US$
Attainable crop
production (2002 prices)
Actual crop production
Production without crop
protection
Losses prevented by crop
protection
Actual annual losses to
world crop production
Losses caused by disease
1–5 trillion
995 billion
445 billion
415 billion
550 billion
220 billion
Rusts damage plants and plant products,
causing economic losses. Crop protection
measures result in increased prices of primary products to consumers and pollution
of the environment. Rusts are also pathogenic to animals and humans. Diseases are
responsible for minor aesthetic losses – in
domestic gardens, avenues and forests. There
are six vital processes in plants and, correspondingly, six ways in which rusts affect
their hosts adversely. Rusts as pathogens
damage foliage, the main organ of photosynthesis, destroy seedlings, impair growth and
interfere in the metabolism of the hosts.
Rusts keep hosts alive and active for their
own growth, development and spread. Hence,
pathogens and hosts have coevolved. At the
same time, rusts are a useful means of controlling weeds as their infection results in
thinning of plants in the field. Artificial infection of rust fungi in fodder grasses brings
about an increase in protein content.
Crop Losses
Green Revolution and Grain
Production in India
Conservative estimates of total annual losses
in crop production by diseases, insects and
weeds worldwide are 220 billion US$ corresponding to 31–42% of all losses, of which
diseases are 14.1%, insects 10.2% and weeds
12.2%, while 6–12% losses are postharvest
losses.
In India, a green revolution began in 1960.
In 1965, hybrid varieties were introduced,
followed by an increased consumption of
fertilizers (N, K, P). From the 53% of total
area under cereal cultivation, hybrid cultivars
have been introduced in 16% of the area.
These innovations in agricultural practices
206
M.S. Patil and A. Patil
revolutionized grain production in India
from 1900 to 1971. From 1900/01 to 1910,
grain production was 67.6 Mt. It then
remained stable until 1948, in the preindependence era. During 1948–1949, there
was a wheat famine. During the first three of
the Five Year Plans (1950–1965), the rate of
grain production increased by 2.5%. In the
fourth Five Year Plan, grain production was
approximately 100 Mt, i.e. grain production
increased by 5%. The food crisis provided
not only a warning but also an opportunity
for new thinking. Still, there is some hope
as the International Grains Council has forecast a 7% increase in global wheat production. India is the world’s second largest
producer of wheat and rice and the expected
wheat harvest this year is 76.8 Mt and rice
production is 95.7 Mt. Among the major
cereal crops, wheat is the most suitable due
to its superior quality of grain, coupled with
its wide adaptability for cultivation under
varied conditions; humans and wheat will
survive in any environment. Even today,
the wheat rust management mission is still
incomplete and awaiting novel solutions to
increase the yield of quality grains. Lord
John Boyd Orr, the first Director of the FAO,
said in 1948, ‘a lifetime of poor nutrition
and actual hunger is the fate of at least 2/3rd
of the world’s population’. This is still true,
even today.
Diseases of Crop Plants and
Associated Pathogens
4. Rust of jowar (Sorghum bicolor (L.)
Moench):
(i) P. purpurea Cke.;
(ii) P. levis Arthur;
(iii) P. nakanishiki Dietel.
5. Rust of pearl millet or bajara (Pennisetum
glaucum (L.) R. Br.):
(i) P. substriata Ell. and Barth var. indica
Ramachar and Cummins, India;
(ii) P. substriata Ell. and Barth. var. decrospora Eboh, Nigeria;
(iii) P. substriata Ell. and Barth. var. penicillaris Ramachar and Cummins.
6. Rust of maize (Zea mays L.):
(i) P. sorghi Schw. (common corn rust);
(ii) P. polysora Underw. (southern corn
rust);
(iii) Physopella zeae (tropical corn rust).
Oilseed crops
1. Soybean rust (Glycine max (L.) Merr.):
Malupa sojae (P. Henn.) Ono, Y. et al.
or Malupa state of P. pachyrhizi H. and
P. Sydow.
2. Groundnut rust (Arachis hypogaea L.):
P. arachidis Speg.
3. Sunflower rust (Helianthus annuus L.):
P. helianthi Schwein.
4. Safflower rust (Carthamus tinctorius L.):
(i) P. carthami Corda;
(ii) P. caleitrapae var. centaureae (DC.)
Cummins.
5. Linseed/flax rust (Linum usitatissimum
L.): M. lini (Ehrb.) Lev.
Grain crops
Plantation crops
1.
Wheat rusts (Triticum spp.)
(i) Black stem rust: P. graminis Pers.
tritici Eriks. and Hennen;
(ii) Brown rust: P. recondita Rob. ex
Desm.;
(iii) Yellow or stripe rust: P. striiformis
West.
2. Leaf rust of rye (Secale cerealis L.):
P. graminis Pers. secalis.
3. Leaf or crown rust of oat (Avena sativa
L.): P. coronata Corda and P. graminis Pers.
avenae Fraser & Ledingham.
1. Coffee leaf rust (Coffea arabica L. and
other spp.):
(i) H. vastatrix Berk. and Br.;
(ii) H. coffeicola (reported only from
Cameroon, West Africa).
2. Mulberry rusts (Morus alba L. and other
spp.):
(i) A. mori Barclay;
(ii) Cerotelium fici (Butler) Arthur.
3. Dalbergia rust (Dalbergia spp.): Sphaerophragmium dalbergiae Dietel = U. dalbergiae
The Rust Fungi
P. Henn. (1895) = U. sisso Syd. & Butl.
(1906).
4. Teak rust (Tectona grandis L.): Olivea
tectonae (Ramkr., T.S. and K.) Mulder =
Chaconia tectonae Ramkr., T.S. and K.
2.
3.
207
Rust of Vitex spp.:
(i) O. fimbriata (Mains) Cumm. & Hirat.;
(ii) O. scitula H. Sydow;
(iii) O. viticis Ono, Y. and Hennen.
Rust of Vinca major L.: P. vincae Berk.
Pulses and vegetables
Ornamental plants
1. Green gram rust (Cicer arietinum L.):
Uromyces ciceris arietini (Gron.) Jack.
2. Rust of Phaseolus sp: U. appendiculatus var. appendiculatus (Pers.) Unger.
3. Cowpea rust (Vigna sp.): U. vignae
Barclay.
4. Bean rusts (Pisum, Vicia, Lens, Lathyrus spp.):
(i) U. viciae-fabae (Pers.), Schroeter,
autoecious rust in Europe and America;
(ii) U. pisi (Pers.) Wint., heteroecious
rust in Europe, rarely in India.
5. Chilli rust (Capsicum annuum L.): P.
pampaeana Speg. (Mexico, Peru, Brazil,
Columbia and Gautemala).
1. Rose rusts (Rosa spp.): Phragmidium
spp. (10 spp.).
2. Gladiolus rust (Gladiolus spp.): P. gladioli (Duby) Cast.
3. Tulip rust (Tulipa spp.): P. prostii
Moug. (on wild species).
4. Canna rust (Canna spp.): P. thalie Dietel.
5. Chrysanthemum rust (Chrysanthemum
spp.): P. chrysanthemi Roze.
6. Saxifraga rust (Saxifraga spp.): P. saxifragae Schlecht.
Forage crops (Fabaceae)
Rust of wheat (Triticum spp.)
1. Bersim/clover rust (Trifolium spp.).
2. Lucerne (Medicago spp.).
3. Alfalfa rust (M. sativus L.): U. striatus
Schroeter.
4. Clover rusts (Trifolium spp.):
(i) U. trifolii-repentis Liro;
(ii) U. fallens (Arthur) Barth.
In India, wheat is cultivated on 16m ha,
but the average yield is very low, that is,
810–1000 kg/ha as compared to the wheat
yield in other countries, namely Argentina
1210 kg/ha, America 1610 kg/ha, Belgium
3700 kg/ha and Denmark 4000 kg/ha. The
five major wheat-growing areas in India are
in the north-western zone, the north-eastern
zone and the central, peninsular and northern hilly zones. Gene exchange of the new
allopolyploids inevitably would have resulted
through hyphal fusion, nuclear exchange and
genetic recombination in urediniospore population. This results in widening the host
range of the hybrid rusts because of gene
diversity and increases the value of survival
and nutritional status of the rust. Continuous introduction of hybrid cultivars in the
field through plant breeding serve as new
hosts to rust. Three species of wheat and their
cultivars are used mainly for cultivation of
durum wheat (T. durum Desf.) or macaroni
wheat, which covers about an 85% area.
Bread wheat (T. aestivum L.) covers a 14%
Fibre crops
1.
Cotton rusts (Gossypium spp.):
(i) P. gossypi (Arthur) Hiratsuka =
U. gossy = C. desmium;
(ii) A. gossypi, an aecial state of P.
cacabata Arthur & Holway.
2. Linseed/flax rust (Linum usitatissimum
L.): M. lini (Ehrb.) Lev.
Medicinal plants
1. Adhatoda zeylanica Nees. rust: Chrysocelis butteri (Dietel and Sydow) Laundon.
The key to species and varieties of Puccinia
and Uromyces producing rust diseases can
be found in the Appendix at the end of this
chapter.
208
M.S. Patil and A. Patil
area and emmer wheat only about a 1% land
area. Rusts of wheat have many physiological races and wheat cultivars are recommended by plant breeders in specific regions
for cultivation to avoid rust disease development. Epidemiological studies of three
rusts have shown that collateral hosts have
a restricted role, while alternate hosts virtually have no role at all. The survival of these
rusts in India is primarily through urediniospores that survive on self-growing plants
or volunteer plants. The airborne urediniospores favoured by wind and rain due to
tropical cyclones in the months of October
and November get dispersed and deposited
over Central India from the Nilgiri Hills in
South India. Brown and black stem rusts
become established there and then subsequently spread to the eastern and northern
states of India over the Indo-Gangetic Plain.
Brown rust appears first in the Himalayan
foothills, eastern Uttar Pradesh and north
Bihar in the month of January.
The western lines associated with disturbances and rain spread the pathogen to
the north-western states of India, along with
yellow rust. The Nilgiri and Pulney Hills
are the primary focal point providing the
source of inoculum, that is urediniospores
migrating upward with air currents towards
the north via Central India, periodically
trapped and studied by Mehta (1929, 1952).
The Mahabaleshwar and Panchgani plateaus
also serve as a focal point for the secondary
source of inoculum (Joshi et al., 1986).
Rust of coffee
Coffee rust is a disease of the coffee plantation crop, namely Coffea arabica L., C. libarica
and C. canephora, cultivated for berries to
produce coffee, a well known non-alcoholic
drink like tea, in Ethiopia, Yemen, Sri Lanka,
South and Central Africa, Cameroon, Bahamas, Brazil and India. The world production
of coffee is 3.16 Mt/year. In India, coffee is
cultivated mainly on the hill slopes of Karnataka, Tamil Nadu and Kerala. Coffee production is estimated to be 964,000 t/year on
a worldwide basis and its production in
India is 230,000 t. The pathogen of the coffee
plant is H. vastatrix Berk. and Br., which
develops very serious disease on foliage,
leading to defoliation.
Rust of groundnut (Arachis hypogaea L.)
Groundnut is the world’s second largest
source of edible oil and ranks 13th in production among world food crops. India is the
largest producer of groundnut. Groundnut is
cultivated in 26m ha of land worldwide and
produces 34.5 Mt/year. Groundnut is cultivated in India on 7.6m ha and produces
7.8 Mt/year. The major states in India cultivating groundnut are Gujarat, Maharashtra,
Tamil Nadu and Andhra Pradesh. This crop
is attacked by 55 pathogens. Among all the
diseases, three diseases, namely groundnut
rust and early and late leaf spot diseases, are
more serious. They generally develop simultaneously and pod yield decreases by up to
10–70% (Ghewande and Savalya, 1999). Rust
is caused by P. arachidis Speg.; this perpetuates by uredinia, the only spore state throughout the world except teleutospores, which
were recorded in Paraguay only once. It is
not known how groundnut rust perpetuates
and it occurs regularly every year in India
without having a telial state, alternate or collateral host. It is said that groundnut rust
develops first in South India and then
migrates to North India. This disease, along
with early and late leaf spot disease, renders
the crop uneconomical in the rainy season,
which is the major period of groundnut cultivation in India.
Rust of jowar (Sorghum spp.)
The genus Sorghum Moench has 23 species
(Simon, 1993). The crop is damaged by four
different fungal diseases: seed and seedling
disease, foliage disease, head disease and
root and stalk disease. Sorghum rust is a foliage disease which infects almost all species
of Sorghum. High temperatures (75–80°F)
and humid weather is favourable for disease
development. The species of Puccinia that
infect Sorghum (Cummins, 1971) are P. purpurea, P. levis and P. nakanishiki. The most
The Rust Fungi
prevalent rust of jowar throughout the world is
P. purpurea Cooke, which is heteroecious and
its aecial host is O. corniculata L. But the aecial
stage plays a negligible role in rust disease.
The rust infection and host reaction results
in the formation of bright purple-coloured
spots on the leaves. There are 32 races in
cultivated Sorghum distributed in Southeast Asia (11) and Africa (21).
Rust of maize (Zea mays L.)
Maize rust or leaf rust of maize, P. sorghi
Schw., is an American rust. However,
maize is susceptible to two more rusts, i.e.
P. polysora, southern corn rust, and P.
zeae, tropical corn rust. It is a heteroecious
rust and its aecia are produced on species of
Oxalis, namely O. stricta, according to
Arthur (1929). Aecia are more common in
this rust than in Sorghum rust. However,
Mishra (1962) has claimed that the alternate
host of maize rust is O. corniculata, on
which aecia were collected from Nepal. The
rust infects all types of maize with a varying
degree of severity.
Rust of bajra/pearl millet
(Pennisetum spp.)
The genus Pennisetum Rich. has c.80 species and is distributed throughout the tropics. Pearl millet (P. glaucum (L.) R. Br.) is a
staple food crop of the semi-arid tropical
parts of the world, mainly Asia and Africa.
There are about 14 rusts reported on bajra
(Cummins, 1971). However, only two are
well-known, namely P. substriata Ell. and
Barth. var. indica Ramachar and Cummins
in India and P. substriata Ell. & Barth. var.
decrospora Eboh., recently reported from
Nigeria. The first rust is predominant in
India and is heteroecious. The alternate
aecidial host is the species of Solanum. Rust
infection produces pustules on both sides of
the leaf with necrotic spots, due to which
premature drying of leaves may result.
Occasionally, pustules also develop on leaf
sheaths and stem.
209
Rust of flax/linseed
(Linum usitatissimum L.)
This crop is cultivated mainly for oil and
fibre. It is affected by rust in most of the
linseed-growing areas of the world such as
Asia, America and Europe. The rust appears
in India in February. This rust, M. lini (Ehrb.)
Lev., also infects wild species of Linum. It is
an autoecious rust and infects all the green
parts of the plant. Telia develop late on stems
and form crusts covered by epidermis. Aecia
of this rust are caeomoid. Flor (1956) studied
this rust and differentiated 179 races from
America alone. Eighteen races are reported
from India. L. mysorense L., a wild host, has
been reported to harbour the rust from India.
Rust of pea (Pisum sativum L.)
Pulse crops are affected by two rusts, namely
U. pisi (Pers.) Wint., a heteroecious rust
reported from Europe only and rarely in India,
while U. viciae-fabae (Pers.) Schroeter, an
autoecious rust, is found in Europe, America
and Asia. It was found that aeciospores played
a major role in the dissemination of lentil rust
during the active growing season. It was also
suggested that secondary aecia are produced
at low temperatures (17–22°C), while higher
temperatures induced uredinia. Teleutospores
are dormant spores, survive for 2 years and
remain viable at low temperatures (3–18°C).
Rust of gram (Cicer arietinum L.)
Gram rust is caused by U. ciceris-arietini
(Gron.) Jack. It is a heteroecious rust, but no
alternate host or pycnia and aecia have been
collected. The same rust has been collected
on wild species of Trigonella polycerata, a
weed of Fabaceae growing at higher altitudes.
It is claimed to be the source of urediniospores, the primary inoculum.
Rust of bean
Beans belong to different genera of the family
Fabaceae, namely Phaseolus, Vicia, Lathyrus,
210
M.S. Patil and A. Patil
Pisum, Dolichos and Vigna. Their commercial cultivars are a source of vegetables,
pulses and forage crops. They are cultivated
extensively all over the world as kharif and
rabi crops. These crops are infected in the
field by many rusts. The pathogens are U.
appendiculatus (Pers.) Unger var. appendiculatus, U. vignae Barclay, U. viciae-fabae
(Pers.) Schroeter and U. pisi (Pers.) Wint.
The first three pathogens are autoecious and
the fourth is heteroecious. Many races of
these pathogens are known. Heavy infection
of leaves results in defoliation and poor
productivity.
Rust of rose (Rosa spp.)
All rusts of roses belong to the genus Phragmidium and ten species infect roses worldwide. However, seven species are very
common. The most common species is P.
disciflorum, which perpetuates on hybrids
of R. canina and R. gallica, but is less
likely to attack climbing or rambling roses
like R. multiflora. The rusts are autoecious
and all spore types, except pycniospores,
are equally harmful to the foliage. As a
result of infection in some seasons, severe
defoliation ensues and plants are greatly
weakened. Keeping a garden clean is the
most effective method of keeping roses
healthy.
Rust of soybean (Glycine max (L.) Mill.)
Soybean as an oilseed crop is cultivated all
over the world as kharif. About 25 diseases
are known on soybean crop. Among these,
there are 19 predominant fungal diseases. In
fungal pathogens, rust is the most serious in
India. The rust entered India in 1970 from
the New World to Japan via Nepal in northern India and spread to the south-western
parts of India up until 1995. The rust was
first reported from Taiwan. The fungus inciting soybean in Asia was first described as
U. sojae P. Henn. from Japan in 1903. It was
subsequently described and renamed by
Sydow, as U. sojae H. and P. Sydow. However, this was erroneous due to the host not
being soybean but Mucuna spp. (Butler and
Bisby, 1931). Moreover, U. sojae P. Henn. is
not considered as an anamorph of U. mucunaei Rabenh. The anamorph and teliomorph
connection was first proved by Sawada
(1931) and named as P. sojae Sawada. In
the two rusts of soybean, namely P. pachyrhizi and P. meibomaiae (Arthur) Arthur,
the former species shows wide geographical
distribution in Asia, Australia and Africa,
while the latter is restricted to America. In
India, soybean rust does not produce telia
and perpetuates only by uredinia, possibly
due to environmental factors.
Rust of fig (Ficus carica L.)
Rust of cotton (Gossypium spp.)
The genus Gossypium is known by 4–5 species. Use of cotton fibre in India is found in
‘Rig Veda’. Cotton cultivation is mainly for
fibre and oil from seeds. Cotton is cultivated
as a cash crop in 80 countries of the world.
The cotton crop is infected heavily by a
large number of pathogens, including rusts.
Among all the rusts, P. gossypi (Arthur)
Hiratsuka is the most troublesome to cotton,
not only in cultivated varieties but also in
perennial cotton. The rust disease seriously
damages the cotton crop and is responsible
for c.20–70% financial loss to cotton growers annually.
Rust of fig and other species of Ficus is produced by C. fici (Butler) Arthur. It is a common rust found throughout tropical and
subtropical parts of the world. Telia have been
observed only in India on F. glomerata Roxb.,
an evergreen shade tree. In the commercial
fig, F. carica L., this rust appears late in the
season (monsoon) and does not affect the
quality of the fruits, but defoliation exposes
them to sunburn. It also reduces host vigour.
It is possible to have a cell-free culture
using a complex culture medium to culture
rust urediniospores. The method was first
used successfully in black stem rust, P.
graminis tritici, for sporulation. Nowadays,
compounds like kinetin and benzimidazole,
The Rust Fungi
which exert a cytokinin effect, are used to
culture detached leaves in solution in test
tubes for up to a month to determine the
races of rusts.
211
‘Deities – rust gods, Robigan and Robigus’.
Thomas Knight, the English plant physiologist, gave experimental proof of the ability
of aeciospore to infect cereals and after that
same year, voluntary eradication started in
Denmark.
Rust Disease Management Strategies
There is a need for effective disease forecasting and warning systems, as well as a disease calendar for each crop. Crop protection/
management varies widely with different
crops, due to different factors such as:
●
●
●
●
varietal susceptibility of crops
soil types
agronomic practices and
cropping patterns.
The study of diseases, disease development,
disease outbreak, pathogens, varieties, races,
biotypes, ecotypes, pathotypes, specialization, host plants, their hybrids, cultivars,
fluctuating factors like soil, water, fertilizers, pesticides, host–pathogen complex, etc.
is very vast and difficult. The outbreak of
disease on an epiphytotic scale resulting
from the interaction of pathogen, host and
environment can be represented through
disease progress curves (DPC). The problem
becomes more serious due to a pathogen
having high pathogenicity, which includes:
●
●
virulence and
aggressiveness (vigorous races).
Today, virulence, based on evidence, can
often be considered to be oligogenic, in which
a few genes are involved, as suggested by
van der Plank (1968), while aggressiveness
is generally polygenically inherited. Virulence is conditioned due to gene diversity
and aggressiveness by variation in the doses
of enzymes. Hence, disease reaction is a
chemical process which entails changes in
the host and parasite cell metabolism. Black
stem rust is found to be more severe on
wheat than barley and yellow rust on barley
than wheat. To avoid heavy losses, early
sowing was recommended by Pliny. In
Greek and Roman civilizations, the appearance of plant diseases was attributed to
Strategy of management of plant
diseases (Sharvelle, Strategy of
Plant Disease Control, 1961)
Most control measures either reduce the initial inoculum or rate of spread of plant
pathogens (van der Plank, 1963). It is important to reduce and delay the initial infection
as much as possible, by disease forecasting
in advance so that farmers can protect the
crop plants to avoid monetary losses. Sharvelle (1961) classified the strategies for plant
disease control into two categories: (i)
immunization; and (ii) prophylaxis (to eradicate the pathogen).
Strategy of plant disease control
Immunization
1.
2.
Genetical resistance.
Induced resistance.
Prophylaxis
1.
2.
3.
Protection:
(i) Chemical prophylaxis;
(ii) Environmental manipulation.
Eradication:
(i) Crop rotation;
(ii) Sanitation;
(iii) Alternate host elimination;
(iv) Chemical eradication.
Legislation:
(i) Quarantine;
(ii) Regulatory measures.
Developing New Strategies for
Disease Management: Role of
Oxidative Burst
The molecular biology of interactions between
disease-resistance genes, defence genes and
212
M.S. Patil and A. Patil
their role in genetic engineered diseaseresistance elicitor (signal) molecules has been
detected in fungal, bacterial and viral pathogens. These molecules serve as signals to
elicit defence mechanism of the host. Host
resistance genes may function as receptors
of these signals. Only a few disease-resistance genes have been cloned from plants.
Analysis of these genes shows the presence
of leucine-rich repeats (LRRS), leucine zippers and nuclear localization signals
(NLS). LRRS are involved in protein–protein interactions of the signal transduction
pathway. Several defence genes are widely
found in both resistant and susceptible
plants and are involved in the production of
antimicrobial compounds, namely phenols,
phytoalexins and pathogen-related (PR) proteins; PR-1, 2,3,5,6 and 8, co-enzyme reductase
(HMGR), transgenic plant expressing phenylalanine ammonial-yases (PAL) showed
enhanced disease resistance (Vidyasekaran,
1997).
Fungicides in the Control
of Rust Diseases
The different fungicide dosages recommended for rust disease control are shown in
Table 16.1.
Other Methods of Plant
Disease Management
Biological methods
Biological control using different microbes
is the most popular and ecologically safe
method, but is not used practically due to
many constraints. The following are some
potential and promising parasites of rusts
which can be used in biological control in
the future.
1. Aphanoderma album (Preuss.) W.Gams
This Hyphomycetes is characterized to produce a metabolite which switches off sporulation of urediniospores to teleutospores, thus
terminating the life cycle of rust.
2.
Cladosporium spp.
(i) C. aecidiicola Theum.;
(ii) C. exobasidii Jaap;
(iii) C. uredinicola Speg. on P. recondita
(UK).
3. Verticillium spp.
(i) V. hemileiae Steyaert;
(ii) V. lecanii (Zimm.) Viegas.
V. hemileiae and V. lecanii, as a virtue of
their growth on uredinia of coffee rust in a
moist environment, produce a chitinase enzyme to weaken the wall of the spores, as a
result of which the spores burst. Even the cultural filtrates are effective (Ellis, 1971, 1976).
4. Sphaerellopsis filum (Biv. – Bern. ex.
Fr.) B.C. Sutton = Darluca filum (Biv.) Cast.
This hyperparasite was considered by Tarr
(1972) as an ecologically balanced mycoparasite on rust fungi, especially the uredinia of the species of Puccinia and Uromyces of grasses. It is distributed in the
tropical and subtropical moist regions of
the world. However, it has not been used
commercially.
5. Tuberculina costaricana H. Sydow.
6. Olpidium uredinis, an endoparasite in
urediniospores.
Plant breeding
1. Cultivation of hybrid cultivars recommended by plant breeders in different regions for different rusts. The hybrids have
high resistance coupled with good quality
and high productivity. Some rust-resistant
cultivars are ‘Maris Ranger’, ‘Heines VII’,
‘Fenman’, ‘Hybrid 46’, ‘Minster’, ‘Opal’, CS
2D/2M, T. spelta 391, T. spelta G652 (ICARDA), CIM 25, ‘Dove’ (CIMMYT), HD 4502,
‘Arkan’, ‘Blueboy II’, ‘Centurk’, ‘Chris’ (USA),
‘Banks’ and ‘Egret’ (Australia).
2. Use of defence activators – spray of salicylic acid (SA), ferric chloride (FeCl3) and
dipotassium hydrogen phosphate (K2HPO4).
3. Adult plant resistance (APR) – there
has been an increasing interest in adult plant
resistance, especially against leaf rust or
brown rust pathogens of wheat in North
India, because of its widespread occurrence
in germplasm and its durability.
The Rust Fungi
213
Table 16.1. The various fungicide dosages recommended for different diseases.
I – Sulphur fungicides
Sulphur
2–4 kg/ha
Spray or dust 2–3 times
Lime – sulphur
Ziram
0.75 g/100 g
0.2% solution
Sprays
Spraying at the outbreak
of disease and repeated
at weekly intervals
Ferbam
0.15% solution
Sprays as required
Thiram
0.3% solution
0.15–0.2% spray
Seed treatment
Sprays
0.2–0.3% solution
0.5% solution
Dry seed dressing
Two sprays at 10–15-day
intervals
Applied at 4-week intervals
from petal fall
3 sprays at 14-day intervals
Sprays at disease outbreak
and repeat after 10–15-day
intervals
5 sprays at 10-day intervals
Applied at 4-week intervals
from petal fall
3 sprays at 15-day intervals
starting in last week of
September before disease
outbreak
5 weekly sprays in the
growing season
Spray at disease outbreak
and repeat at c.10-day
intervals as necessary
Sprays at disease outbreak
and repeat at 7–10-day
intervals
Sprays at 14-day intervals
along with benomyl or
carbandazim
4 sprays at 10-day intervals
from disease outbreak.
Add triton at rate of 2 ml/l
suspension
Seed treatment
4 sprays at 12-day intervals
Sprays
Dusting 3–6 applications
at 10-day intervals
Sprays 4–5, beginning when
disease appears
Zineb
0.2% solution
0.2% solution
0.15% solution
0.2% solution
0.2% solution
0.2% solution
0.2% solution
0.2% solution
Maneb/
mancozeb
0.2% solution
0.25% solution
0.2% solution
0.3% solution
0.2% solution
400 ppm
2–3 kg/ha
Nabam
0.15–0.25%
solution
Rust of beans – Uromyces
appendiculatus var.
appendiculatus, U. fabae
Bean rust – U. fabae
Bean – U. appendiculatus or
U. phaseoli
Mint – Puccinia menthae
Sunflower – P. helianthi
Rose – Phragmidium
mucronatum
Safflower – P. carthamii
Apricot – Tranzschelia discolor
Plum – T. discolor
Safflower – P. carthamii
Beet – U. betae
Almond and apricot – T. discolor
Barley – P. striiformis
Chrysanthemum –
P. chrysanthemi
Garlic – P. allii
Peach – T. discolor
Phalsa – Dasturiella grewiae
Soybean – Phakopsora
pachyrhizi
Wheat – P. recondita and
P. graminis tritici
Bean – U. appendiculatus
var. appendiculatus
Groundnut – P. arachidis
Peas – U. fabae
Safflower – P. carthamii
Sorghum – P. purpurea
Sunflower – P. helianthi
Wheat – P. graminis tritici and
P. recondita
Wheat – P. graminis tritici and
P. recondita
continued
214
M.S. Patil and A. Patil
Table 16.1. continued.
II – Copper fungicides
Bordeaux
mixture
5:5:1/2:50
Copper
oxychloride
1.8 kg/400 l
Colloidal copper 1500 ml/400 l
0.35% solution (kocide 110)
and 0.5% (cupravit)
Sprays, 5–10
applications, add urea
and ZnSO4
Sprays
Sprays
Sprays in March–April
300 l/ha
Coffee – Hemileia vastatrix
Beans – U. appendiculatus
Beans – U. appendiculatus
Coffee – H. vastatrix
III – Mercury fungicides
Agrosan GN
0.3% solution
Seed treatment
Safflower – P. carthamii
IV – Heterocyclic nitrogenous compounds
Captafol
0.3% solution
0.2% solution
Seed treatment
3 sprays at 10-day intervals
as soon as disease appears
Safflower – P. carthamii
Brown rust of wheat:
P. recondita
V – Systemic fungicides
Benzimidazole
1.5 kg/ha
Oxathiin-carboxin
1 g/kg
2.5 g/kg
Oxycarboxin
2630 g/100 kg
Dust or spray with carbendazim or
carbendazim (0.07%) plus mancozeb
(0.15%)
Seed dressing with benomyl
Seed treatment/foliar spray and
soil treatment
Seed treatment
1.12 kg/ha
1.68 kg/ha
3 kg/ha
Granules in soil
Spray
Spray – single in Europe or
2–3 sprays common
15 kg/ha
Soil treatment
Soil treatment with benomyl spray
Soil treatment with benomyl as spray
Groundnut – P. arachidis
Speg. and early and late
leaf spot disease
Safflower – P. carthamii
Bean –
U. appendiculatus
Stripe rust of wheat –
P. striiformis
Leaf rust wheat –
P. recondita
Black stem rust –
P. graminis tritici and
P. recondita
Safflower (seedling)
stage – P. carthamii
Fig rust – Cerotelium fici
Peanut – P. arachidis
Sunflower – P. helianthi
VI – Benzanilide derivatives
Benzanilide
1.87 kg/ha
Foliar spray
Benodanyl/
Vitavax
500 mg/ml
2 sprays or soil drenching
0.75 l/ha
0.3%
250 ml/l
0.1125%
100 mg/l
10 g/kg
0.2%
Spray/seed treatment
4 sprays at 15-day intervals
2 sprays or soil drenching
Sprays at 6–9-day intervals
2 sprays or soil drenching
Spray/seed treatment
Spray/seed treatment
Oxycarboxin
Plantavax w.p.
Stripe rust of wheat – P. striiformis
and barley – P. hordei
Effective against most of the
rusts of cultivated plants
Stripe rust of wheat – P. striiformis
Leaf rust of wheat – P. recondita
Sunflower – P. helianthi
Sunflower – P. helianthi
Sunflower – P. helianthi
Stripe rust of wheat – P. striiformis
Black stem rust – P. graminis tritici
The Rust Fungi
215
Integrated disease management
Acknowledgement
1. Cultural methods include early sowing.
2. Spraying of micronutrients such as
Na2B4O7, CuSO4 increases resistance in
plants.
The authors express their gratitude to The
Principal, Agriculture College, Kolhapur,
India, for providing the facilities of their
library.
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M.S. Patil and A. Patil
Appendix 1
Key to species and varieties of Puccinia and Uromyces producing rust diseases to
cultivated crops which are very common but confusing to identify
Wheat rusts
I.
I’.
II.
II’.
Uredinial infection produces chlorotic streaks with halos on leaves -----------P. striformis
Uredinial infection does not produce chlorotic streaks with halos on leaves ----------- II
Urediniospores with 5–6 germ pores arranged equatorially -------------P. graminis tritici
Urediniospores with 4–5 germ pores, scattered ----------------------------------- P. recondita
Jowar rusts
I.
I’.
II.
II’.
Uredinia aparaphysate ---------------------------------------------------------------------------- P. levis
Uredinia paraphysate -------------------------------------------------------------------------------------II
Urediniospores with 5–8 germ pores, scattered ---------------------------------P. nakanishiki
Urediniospores with 3–5 germ pores, equatorial ---------------------------------- P. purpurea
Bajra rusts
I. Infection mostly hyphophyllous, teleutospores measure 21–49 µm long, 2-celled, pedicel coloured and short ---------- P. substriata Ell. & Barth. var. indica Ramachar & Cummins
I’. Infection amphigenous, teleutospores large, up to 5-celled ---------------------P. substriata
Ell. & Barth. var. decrospora Eboh.
Maize rusts
I. Teleutospores stalked and 2-celled -------------------------------------------------------------------II
I’ Teleutospores sessile, in chain and innate -------------------------------------------------------- P. zeae
II. Telia exposed/erumpent, urediniospores 26–31 µm long (aecia on Oxalis stricta L.)-------------------------------------------------------------------------------------------------------------------- P. sorghi
II’. Telia covered, urediniospores 29–30 µm long (aecia not known) -------------- P. polysora
Rusts of Phaseolus and Vigna spp.
I. Urediniospores measure 24–29 × 17–19 µm, germ pores 4, equatorial (on Phaseolus
spp.) ------------------------------------------- U. appendiculatus (Pers.) Unger var. appendiculatus
I’. Urediniospores measure 29–32 × 20–22 µm, germ pores 2, equatorial (on Vigna spp.)
-------------------------------------------------------------------------------------------------------------- U. vignae
Rusts in forage crops
I.
Urediniospores have 4–7 germ pores, scattered (on red clover) ------------------U. fallens;
T. pratens (only uredia and telia)
I’. Urediniospores have 2–4 germ pores, equatorial (on white clover) --------------------------------------------------- U. trifolii-repentis T. repens and other spp. autoecious and macrocyclic
Rusts on vegetable and pulse crops
I.
I’.
Autoecious, occurs in America and Europe ------------------------------------ U. viciae-fabae
Heteroecious, occurs only in Europe (Arthur, 1929) on P. sativum L. -------------- U. pisi
17
Etiology, Epidemiology and
Management of Fungal Diseases
of Sugarcane
Ayman M.H. Esh
Biotechnology and Tissue Culture Laboratories, Sugar Crops Research Institute,
Agricultural Research Center, Giza, Egypt
Abstract
Sugarcane (Saccharum officinarum L.) is one of the most important commercial crops in many countries of the world. It contributes nearly 70% of world sugar and provides the base materials essential
for many other industries. Sugarcane crop is attacked by numerous foliar and root pathogens. Some of
these diseases cause serious quantitative and qualitative losses which have negative effects on sugarcane production, as well as in the sugar industry.
About 56 diseases of sugarcane have been reported so far from different parts of the world. Of
these, 40 are caused by fungi, several of which can cause economic losses. The major sugarcane fungal
diseases in different tropical and subtropical regions are: smut disease (Ustilago scitaminea); rust disease (Puccinia melanocephela); red rot (Glomerella tucumanensis [Colletotrichum falcatum]); eye
spot disease (Bipolaris sacchari), pokkah boeng disease (Fusarium moniliforme); and pineapple disease (Ceratocystis paradoxa). This chapter includes the major fungal diseases of sugarcane and the
various control practices used against them.
Introduction
Sugarcane (S. officinarum L.) is a monocotyledonous plant from the family Poaceae
of the subfamily Andropogoneae (Cox et al.,
2000) and is considered as one of the oldest
cultivated crops known to man. Sugar, along
with honey, is the oldest natural sweetener
(Peng, 1984; Naik, 2001). Sugarcane is grown
in the tropical and subtropical regions of
the world and is cultivated in nearly
60 countries as a commercial crop, with
Brazil, India, China, Cuba, Thailand and
Pakistan as the major sugarcane-growing
countries (FAO, 2005).
Due to its wide range of adaptability, it
supplies more than 60% of world sugar
demand and basic raw material in many
industries, which makes it one of the most
important cash crops that plays an enormous role in the economy. Various biotic
and abiotic factors are responsible for yield
reduction and economic losses. Among these
factors, fungal diseases are the major cause.
Over 100 fungi, 10 bacteria, 10 viruses and
about 50 species of nematodes are pests of
sugarcane in different parts of the world
(Singh and Waraitch, 1981). Sugarcane,
being a long duration crop (10–12 months),
remains in the field for several years (ratoons
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
217
218
A.M.H. Esh
remain in the field for up to more than
6 years). A serious drawback of this practice, however, is that pathogens may build
up within the field and be disseminated during propagation of new seed cane. The pathogen within the seed pieces is transmitted
easily into young plants, which in turn serve
as sources of inoculum for secondary infections in adjoining healthy plants. Several
management strategies have been developed
as a result of research and development
work. The endless struggle between varieties
and the complexity of disease have led correspondingly to the development of a variety of approaches for control. The role of
fungicides in modernizing and changing the
condition of agriculture is quite significant
(Mehta, 1971; McFarlane et al., 2006).
Sugarcane Smut Disease
Sugarcane smut, caused by the Basidiomycetes fungus Ustilago scitaminea Syd., is
cosmopolitan in distribution and has been
an important disease in nearly every sugarcane-producing country of the world. It can
reduce crop yields by over 50% and make
ratoon crops unprofitable to maintain. It is
highly infectious and even developed countries have been unable to stay smut free with
the use of appropriate quarantine measures
(Antony, 2008). The disease was first noted
in South Africa in 1877, then in the early
1930s it caused severe problems in India
and other countries in Asia. Years later, the
disease started to establish and cause serious problems in different parts of the world:
1943, Argentina, (Cross, 1960); 1950, Brazil
and Paraguay; 1957, Bolivia; 1960 and 1971,
Hawaii (Byther et al., 1971); and 1974, Guyana (James, 1976). By 1981. the disease had
been found in most of the Caribbean and
North, South and Central America (Ferreira
and Comstock, 1989). In 1998, the disease
was reported for the first time in the Ord
River area of Western Australia (Riley et al.,
1999). Australia is a major exception since
the disease is present only in Western Australia. The sugar industries of eastern Australia, Fiji and Papua New Guinea are still free
of the disease (Braithwaite et al., 2004b).
Causal agents
The fungus belongs taxonomically to Phylum:
Basidiomycota; Class: Ustilaginomycetes,
(Bisby et al., 2007). Classification of U. scitaminea H. & P. Sydow, the causal agent of
sugarcane smut disease, is based mainly on
differences in spore morphology and the
characteristics of germinating spores (LeeLovick, 1978).
Smut races have been reported subsequently based both on observations and
inoculation studies (Gillaspie et al., 1983).
Usually, races are suggested when a cultivar
succumbs to smut after being grown for several years without being infected (James,
1976). U. scitaminea races have been
reported in Hawaii, Pakistan, the Philippines and Taiwan; the presence of the actual
number of races and their prevalence are
unknown (Ferreira and Comstock, 1989).
Pathogenic races of sugarcane smut have
been observed in several countries, including two races A and B from Hawaii (Comstock and Heinz, 1977) and three races (1, 2,
3) reported in Taiwan (Leu and Teng, 1972;
Lee et al., 1999). However, Ferreira and
Comstock (1989) considered the true prevalence of races to be controversial. Many
claims are based on the reaction of the same
cultivar in different countries, but the interpretation of these claims is confused by testto-test variation and the use of different
inoculation methods. Two international
collaborations have attempted to standardize
race typing. Gillaspie et al. (1983) performed
race typing under glasshouse conditions to
standardize the environment and six races
were identified. Grisham (2001) coordinated
a race typing study in nine countries using
local isolates tested against a standardized
set of 11 differential cultivars. On the molecular level, many researchers have studied
the genetic diversity among U. scitaminea
isolates, either between local isolates or
between isolates collected from different
parts of the world (Braithwaite et al., 2004a,b;
Xu et al., 2004; Singh et al., 2005). Genetic
variation estimated from 12 AFLP primer
combinations showed that, overall, there
was little variation in the smut population
across the world. However, isolates from
Fungal Diseases of Sugarcane
the Philippines, Taiwan and Thailand form
a distinct cluster; it is therefore suggested
that genetic variation is limited between the
isolates and the phylogeny of U. scitaminea
is poorly understood (Braithwaite et al.,
2004b).
Disease symptoms
Smut-infected plants are distinguished by
the emergence of a ‘smut whip’. The whips
are the flowering structures of the pathogen which produce teliospores. The flowering structures transform into a whip-like
sori that grows out between the leaf sheaths.
At first, it is covered by a thin silvery
peridium (this is the host tissue), which
peels back easily when desiccated to expose
the sooty black-brown teliospores. Whips
begin emerging from infected cane by
2–4 months of age, with peak whip growth
occurring at the 6th or 7th month. Spindle
leaves are erect before the whip emerges.
Affected sugarcane plants may tiller profusely, with the shoots being more spindly
and erect with small narrow leaves (i.e. the
cane appears ‘grass-like’). Less common
symptoms are leaf and stem galls and bud
proliferation (Ferreira and Comstock, 1989;
Agnihotri, 1990).
Pathogenesis
Ustilago scitaminea produces diploid spores
called teliospores. When teliospores germinate, they undergo meiosis, which gives rise
to a septate promycelium bearing four haploid sporidia (basidiospores). U. scitaminea,
like most parasitic Heterobasidiomycetes,
has a diallelic bipolar mating system (Alexander and Srinivasan, 1966; Leu, 1978;
Moosawi-Jorf et al., 2006) in which only
sporidia of opposite mating types conjugate.
Of the four initial sporidia or basidiospores
from each teliospore, two have a positive
mating allele and two have a negative mating allele. U. scitaminea can thus both selfand outcross, but the frequency of natural
selfing versus outcrossing is unknown. A
219
dikaryotic mycelium develops after fusion
of compatible sporidia. This dikaryotic mycelium is infectious, penetrates behind bud
scales and invades the meristematic zone of
the bud. Entry into the meristem in the bud
occurs between 6 and 36 h after the teliospores are deposited on the surface (Alexander and Ramakrishnan, 1980). Finally, the
apical meristem of smut-infected cane produces a long whip-like structure bearing
billions of teliospores (i.e. sorus).
Sugarcane smut is spread by spores
which have an aerial dispersal mode. The
whip serves as a source of spores that release
approximately one billion spores/whip/day
into the air to infect the buds of the standing
sugarcane. The infected buds remain dormant until the cane is cut for seed. The
spores mixed with the soil of cropped or
newly prepared fields also become a source
of infection to the disease-free seed pieces.
Under normal soil moisture, the spores only
survive for a short time in the soil. On the
other hand, several species of insects have
been associated consistently with smut
whips; this suggests insects could play a
role in spore dispersal (Ferreira and Comstock, 1989; Agnihotri, 1990).
Disease control
The best control method is to use resistant
cultivars. There is a strong genetic basis
for resistance and resistant varieties have
been readily available and used to control
outbreaks of smut in several countries
(Churchill et al., 2006). Disease-free planting material usually can be obtained by subjecting seed to hot water treatment. Hot
water treatment, however, may not be practical on a large scale and its effectiveness
may be subject to varietal differences
(McFarlane et al., 2007).
Several fungicides (triadimefon, fludioxonil:mefenoxam:azoxystrobin, mancozeb,
metalaxyl + carboxin + furathiocarb, pyroquilon, benomyl and chlorothalonil) have been
used to control sugarcane smut when used
as pre-planting fungicidal dips of planting
setts (Wada et al., 1999; Wada, 2003).
220
A.M.H. Esh
Sugarcane Rust Disease
(Common and Orange Rust)
Throughout the world, the important leaf
rust disease causes severe losses in sugarcane fields (Magarey et al., 2008). In 2000,
sugarcane rust was once considered a minor
pathogen in the Australian sugar industry.
In 2000, it devastated most plantations of
the cultivar Q124 in Australia, causing yield
losses of up to 40% (Apan et al., 2003; Braithwaite et al., 2004a; Magarey et al., 2008). In
the USA, the yield loss caused by a rust epidemic due to cultivar CP 72-1210 in 1987
was 20% (Raid and Comstock, 2000).
Sugarcane rust is caused by two species
belonging to the genus Puccinia, P. melanocephala and P. kuehnii, the former causes
common rust disease, while the later causes
orange rust. Common rust caused by P. melanocephala H. & P. Syd. was first reported
on sugarcane in 1949 in the Deccan area in
India (Patel et al., 1950). The disease has
been also reported from: Japan (Ohtsu, 1975
[cited by Muta, 1987]); the Philippines (Serra
et al., 1983); Australia (Egan and Ryan,
1979); Taiwan (Hsieh et al., 1977); Dominican Republic (Presley et al., 1978); Jamaica
(Burgess, 1979); Puerto Rico (Liu, 1979);
Cuba (Sandoval et al., 1983); Carribean and
Central America (Purdy et al., 1983); Hawaii
(Comstock et al., 1982); and Angola, Kenya,
Madagascar, Tanzania, Uganda, Zambia,
Zimbabwe, South Africa, Mozambique and
Malawi (Egan, 1980; Sivanesan and Waller,
1986).
Orange rust caused by P. kuehnii Butl.
was first reported on sugarcane in Java in
1890 (Ryan and Egan, 1989). The disease
has been reported from Japan (Ito, 1909), Australia, Indonesia, the Philippines, Taiwan,
Pacific Islands, Sri Lanka, Malaysia, Thailand,
New Caledonia, China (Egan, 1980; Sivanesan and Waller, 1986) and India (Mukerji
and Bhasin, 1986).
Disease symptoms
The initial symptoms of common rust are
small, elongated yellowish spots, which are
visible on both leaf surfaces. The spots
increase in size up to 1.5 mm in diameter and
usually turn brown to orange-brown or redbrown. The lesions occur irregularly and
typically range from 2 to 10 mm in length,
but occasionally reach 30 mm. The spots
are raised and are surrounded by a pale yellow halo (Raid and Comstock, 2000). The
raised pustules are formed predominantly
on the undersurface of the leaves and the
urediospores formed therein are orange to
orange-brown. On a highly susceptible variety, considerable numbers of pustules may
occur on a leaf, coalescing to form large,
irregular, necrotic areas. High rust severities
may even result in premature death of young
leaves. Severe rust has caused reductions in
both stalk mass and stalk numbers (Rao
et al., 1999; Raid and Comstock, 2000).
Causal agent
Puccinia melanocephala Syd. and P. Syd.
(common rust) and P. kuehnii Butler (orange
rust) are reported to cause rust diseases
on sugarcane (Butler, 1914; Cummins and
Hiratsuka, 1983; Shine et al., 2005; Ido et al.,
2006; Comstock et al., 2008; Ovalle et al.,
2008).
The two obligate parasitic fungi belong
to Phylum: Basidiomycota; Class: Urediniomycetes, Order: Uredinales. The causal agents
of common and orange rust cannot be clearly
distinguished based on colour of lesions
and uredinia and the size of urediniospores.
However, they are distinguishable based on
the presence or absence of abundant capitate paraphyses in uredinia, echinulation,
colour and wall thickness of urediniospores,
colour of the telia and colour and wall thickness of teliospores. P. melanocephala has
abundant capitate paraphyses in uredinia
and urediniospores with dense echinulation, darker brown and uniformly thick
walls. They also have dark brown to blackish
telia with brown to dark brown teliospores
with apically thickened walls. P. kuehnii
has morphologically indistinct paraphyses
in uredinia and urediniospores with
moderate echinulation, lighter brown and
Fungal Diseases of Sugarcane
sometimes apically or uniformly thickened
walls (Virtudazo et al., 2001).
Pathogenesis
The life cycle of sugarcane rust is simple,
with the urediniospore being the only known
infectious spore. These are produced in,
and are released from, pustules that develop
on the underside of sugarcane leaves. The
development of substomatal vesicles, infectious hyphae, haustoria and subsequent
infection processes are similar to other Puccinia spp. Urediniospore production occurs
8–18 days after the initial urediniospore
lands on a leaf, depending on varietal susceptibility and environmental conditions
(CABI CPC, 2006).
Spread of rust disease occurs primarily
by wind and water-splash movement of
urediniospores. The movement of diseased
vegetative parts of sugarcane, contaminated
equipment and workers from one location
to another may also provide a means of
spread. The expression of the disease is influenced by the interaction of genetic, environmental (primarily air temperature and leaf
wetness) and physiological (age of infected
plants) factors. The infection may occur
within the temperature range of 5–34°C; however, the optimal temperatures for spore germination are between 15° and 30°C. Heavy
rains tend to remove spores from the atmosphere, rendering them infective if they
land on the soil (Egan, 1964; Comstock and
Ferreira, 1986). On the other hand, it has
been found that rains favour the development
of orange rust but inhibit the development of
common rust (Croft et al., 2000). Sugarcane
plants appear to be most susceptible at
3–6 months old (Ryan and Egan, 1989).
Disease control
The best control of sugarcane rust is use of
resistant sugarcane varieties. The development of resistant cultivars has decreased
the economic losses caused by this disease
(Ryan and Egan, 1989). Nevertheless, existing
221
resistant cultivars are threatened by the
establishment of new races of the pathogen.
For example, cultivar CP 78-1247 was considered to be resistant or moderately resistant until 1988, and then it exhibited
extremely high rust susceptibility throughout south Florida (Raid, 1989). However,
resistance has not been stable or durable on
certain varieties, presumably because of rust
variants. For this reason, it is highly recommended that growers should diversify their
varietal holdings (Raid and Comstock,
2000). Chemicals like propiconazole/mancozeb, cyproconazole, triadimefon and triadimenol have been used for the control of
sugarcane rust. Several soil factors influence rust infection levels on sugarcane significantly. Studies have shown that rust
levels are higher on sugarcane grown on
low pH soils, high soil moisture and high levels of phosphorus and potassium nutrients
present in the soil (Johnson et al., 2007).
Red Rot Disease of Sugarcane
Red rot is one of the oldest known diseases
of sugarcane. It occurs in most cane-growing
countries. The disease was first described
from Java by Went (1896) and then the disease was reported from Australia, India,
Hawaii and the USA. It is clear that the
disease was widely distributed before the
knowledge of its impact on sugarcane crop
(Singh and Singh, 1989).
Symptoms
The pathogen, Colletotrichum falcatum
Went, can attack any part of the sugarcane
plant – stalk, leaf, buds or roots – but it is
usually considered a stalk and a seed-piece
disease. C. falcatum completes its life cycle
on the sugarcane leaf and usually the damage to the leaf does not pose a serious threat
to cane or cause much harm to the plant
(Singh and Singh, 1989; Raid, 2006).
The most damaging phase of this disease occurs when the pathogen attacks the
stalk. Depending on the age of the stalk,
222
A.M.H. Esh
time of infection and susceptibility of the
cane genotype, it produces different types
of symptoms. The typical stalk symptoms,
that is, presence of white spots in otherwise
rotten (dull red) internodal tissues and
nodal rotting, appear when the crop is at the
fag end of the grand growth phase in subtropical areas. These white patches are specific to the disease and are of significance in
distinguishing red rot from other stalk rots.
At a later stage, some discoloration of rind
often becomes apparent when internal tissues have been badly damaged and are fully
rotten (Singh and Singh, 1989; Raid, 2006;
Duttamajumder, 2008).
In susceptible varieties, the red colour,
sometimes along with some grey colour,
may be seen throughout the length of the
stalk. The infection is confined largely to
the internodes in resistant varieties. On the
leaves, the pathogen may produce elongated
red lesions on the midribs, reddish patches
on the leaf sheaths and, infrequently, small
dark spots on the leaf blades. Eventually,
the lesions may develop a straw colour in
the centre. In seed pieces, the entire seed
piece may become rotten and the internal
tissues turn various shades of red, brown or
grey (Singh and Singh, 1989).
Causal agent
The fungus causing red rot of sugarcane is
commonly known by its imperfect state, i.e.
C. falcatum Went (Glomerella tucumanensis). The perfect state of the fungus belongs to
Phylum: Ascomycota; Class: Ascomycetes;
Family: Glomerellaceae; Genus: Glomerella
(Bisby et al., 2007).
Conidia are falcate (but not markedly
so), fusoid, apices obtuse, 15.5 (25–26.5)–
48 µm × 4 (5–6)–8 µm and contents are
granular and sometime contain oil globules.
At least two races have been identified. The
variations in the asexual state of the fungus
(Colletotrichum state) may originate through:
(i) heterokaryosis; (ii) by recombination
through parasexual mechanism; and (iii) by
the universal mechanism of mutation, selection and adaptation in response to the changes
in the host environment. Heterokaryosis is
the mechanism through which the fungus
collects and consolidates two or more genetically different nuclei in the hypha and
derives the benefit of the introduced genetic
material. These newly gathered nuclei also
multiply in tandem with the native nuclei
(Duttamajumder, 2008).
Pathogenesis
The pathogen mainly infects the stalks
through the nodes. Once the infection is
established in the stalk, the fungal mycelium grows intracellularly and is sparse in
the reddened areas. The dead cells are packed
in white patches with profuse hyphae. The
size and number of these white areas are
correlated with the susceptibility of the
variety. The lesions become dark red, narrow and sharp margins, with a few white
spots in resistant varieties, while in susceptible varieties the lesions become wide,
light red and ill-defined margins with prominent white spots (Singh and Singh, 1989).
One of the major sources of inoculum is
midrib lesions. Also, diseased stalks and
crop debris and infected plant material are
important sources of inoculum and cause
secondary infections. Wind, rain, heavy
dews and irrigation water play a role in the
dispersal of the inoculum. The pathogen
spores washed into the soil may produce
infection in planted seed pieces. Climatic
factors affect both the spread and severity of
red rot. In newly-planted cane, the disease
is favoured by excessive soil moisture,
drought conditions and low temperatures.
Disease control
The use of resistant varieties is the most effective method of prevention and control of
sugarcane red rot disease (Singh and Singh,
1989; Raid, 2006; Singh et al., 2008b). Management of the disease by the use of diseasefree seed canes for planting is impractical
due to the difficulty in diagnosing dormant
infections of the fungus in seed canes under
Fungal Diseases of Sugarcane
field conditions (Viswanathan and Samiyappan, 2002).
It is difficult to manage red rot through
chemotherapy because the impervious
nature of rinds and fibrous nodes at cut
ends does not allow sufficient absorption in
setts (Agnihotri, 1990). However, better crop
stands have been achieved from enhanced
germination obtained by treating seed
pieces with a fungicide before planting
(Raid, 2006).
Thermotherapy (moist hot air or hot
water) is thought useful for inactivation of
red rot pathogen, but it is difficult to remove
deep-seated infections. It is limited in checking secondary infections (Singh, 1973; Singh
and Singh, 1989; Raid, 2006). In India,
extensive studies about the possibilities of
using biological control to control sugarcane red rod disease have been carried out
(Mohanraj et al., 1999). Seventy-five per
cent of canes may be protected against secondary infection of red rot by dipping the
setts for 15 min in 2.5% culture filtrate of
Trichoderma harzianum (Th 38) and also
by applying Trichoderma multiplied culture in press mud 20 kg/ha beneath the setts
in furrows. Besides the biological control of
red rot, the growth in improved resulting is
to enhanced yield by 15.4 t/ha (Singh et al.,
2008a,b).
Sugarcane Eye Spot Disease
Eye spot has been reported in many
sugarcane-growing areas of the world. The
disease was first described by van Breda de
Haan in Java (1892, cited in Comstock and
Lentini, 2005). The disease is prevalent and
is found in 66 sugarcane-growing countries
(Agnihotri, 1990). Generally, the disease
has a minor economic impact on sugarcane
yield in most areas because of the use of
resistant varieties (Comstock and Lentini,
2005). In India, in 1976, the disease was in
epidemic form and affected 1600 ha sugarcane crop in Mandya district of Karnataka
only (Kumaraswami and Urs, 1978). The disease can reduce sugarcane yield by 15–20%
(Sharma et al., 2004).
223
Disease symptoms
Typical mature eye spot symptoms are characterized by a reddish-brown elliptical lesion
(0.5–4.0 mm long, 0.5–2.0 mm wide) with
yellowish-brown margins. Reddish-brown
to yellowish-brown streaks, sometimes
called ‘runners’, extend upward from individual lesions toward the leaf tip. These
streaks are 3–6 mm wide and 30–90 cm
long. The entire leaf eventually may become
necrotic (Comstock and Steiner, 1989; Comstock and Lentini, 2005).
Causal agent
Eye spot disease is caused by Bipolaris
sacchari (Butler) Shoemaker. The fungus
belongs taxonomically to Phylum: Ascomycota; Class: Ascomycetes; Order: Pleosporales. B. sacchari is the teleomorphic stage
of Helminthosporium sacchari Butler. The
name H. sacchari is still used occasionally.
Pathogenesis
Sugarcane eye spot fungus B. sacchari (H.
sacchari) produces a host-specific toxin
(HST). HSTs are a group of structurally
complex and chemically diverse metabolites produced by plant pathogenic strains
of certain fungal species and function as
essential determinants of pathogenicity or
virulence. HSTs are referred to as ‘host
selective’ because they are typically active
only toward plants that serve as hosts for
the pathogens that produce them and disease never occurs in the absence of toxin
production (Wolpert et al., 2002). The HST
is a mixture of three isomers (A, B and C)
(Lesney et al., 1982; Livingston and Scheffer, 1984) and the HST produced by the sugarcane red rot pathogen is responsible for
the disease symptoms (Steiner and Byther,
1971). The fungus causes eye-shaped lesions
on the leaves, followed by the development
of reddish brown streaks or ‘runners’
extending from the lesions toward the tip of
224
A.M.H. Esh
the leaf. The toxic compound from the fungus causes the runner (Steiner and Byther,
1971).
Eye spot spores, which are produced
abundantly on leaf lesions, are dispersed by
wind and rain. High humidity and dew formation are the favoured conditions for
spore germination. The disease is not transmitted by seed pieces and mechanical transmission by equipment and by humans is
unimportant.
a ladder-like appearance. These lesions sometimes break through the surface of the rind,
causing curvature and distortion of the stalk.
Exaggerated versions of these depressions
may look like neatly made ‘knife-cuts’ in
the stalk. In the most advanced stage of pokkah boeng, the entire top (growing point) of
the plant dies (referred to as ‘top rot’). The
ladder-like lesions are due to rupturing of
the diseased cells that cannot keep up with
the growth of the healthy tissue (Martin
et al., 1989; Raid, 2009a).
Disease control
Causal agents
The only practical and efficient method of
control of eye spot disease is with resistant
clones. Chemical control using foliar fungicides is not practical (Comstock and Lentini,
2005).
Pokkah Boeng Disease
Pokkah boeng, which is a potentially
destructive disease of sugarcane, is caused
by Gibberella moniliformis (Sheldon) Wineland. There have been many reported outbreaks of the disease which have been severe,
like the Java outbreak in 1896, but they have
caused little economic loss (Martin et al.,
1989).
Disease symptoms
In the early stages of infection, the symptoms of the disease are chlorotic areas at the
base of young leaves, distortion (wrinkling
and twisting) and shortening of the infected
leaves and finally stalk death in severe cases.
The infected leaves can be distinguished by
their narrow base. Irregular reddish stripes
and specks develop in the chlorotic parts
which appear in mature leaves.
The infection is present in the stalk and
dark reddish streaks may be found extending through several internodes. Also, in
the internodes, the infection may form long
lesions with cross-depressions that give them
The disease is caused by the fungi, Fusarium
moniliforme (G. fujikuroi) and F. moniliforme
var. subglutinans (G. subglutinans). The perithecia of G. fujikuroi occur only on dead plant
material, while the perithecia of G. subglutinans are rarely formed in nature; thus, the perithecia of these two pathogens are rarely
associated with infected sugarcane plants
(Martin et al., 1989). The pathogens have a
wide host range, i.e. rice, corn, sorghum and
many other grasses. These fungi also cause
other diseases, such as seedling blight, scorch,
stalk rot, root rot and stunting in different
crops (Martin et al., 1989; Raid, 2009a).
Pathogenesis
The pathogens of pokkah boeng disease are
transmitted by the movement of spores from
one locality to another by air currents (Martin
et al., 1961; Raid, 2009a,b). Pokkah boeng
disease of sugarcane may also spread from
seeds contaminated with the fungus (Martin
et al., 1961). It appears to be favoured by
dry climatic conditions being followed by a
wet season. Cane that is 3–7 months old and
growing vigorously appears to be most susceptible (Martin et al., 1989).
The pathogen spores enter the spindle
along the margin of a partially unfolded
leaf, then germinate and grow into the inner
tissue of the spindle leaves. The conidia germinate and the mycelium can pass through
the soft cuticle of young leaves to the inner
Fungal Diseases of Sugarcane
tissues because the epidermal tissues are
still fragile and not protected by the plant
system (Dillewijn, 1950). The mycelium
spreads to vascular bundles of the immature
stem and blocks the vessels, which eventually leads to growth distortions and rupture,
and this development shows the ladder-like
lesions (Holliday, 1980; Martin et al., 1989).
Disease control
The only efficient control for pokkah boeng
disease is the use of resistant varieties. Sugarcane resistance to pokkah boeng has been
shown to be highly heritable (Martin et al.,
1989).
Pineapple Disease of Sugarcane
Pineapple disease is an economically important sugarcane disease that is widely distributed in almost all the regions where
sugarcane is grown (Wismer and Bailey,
1989). In India, the disease has been noticed
on different sugarcane varieties (Singh et al.,
1990). The disease is caused by the fungus
Ceratocystis paradoxa, which induces seedpiece decay following planting. The affected
setts emit a smell resembling that of the
mature pineapple fruit (Went, 1896).
225
acetate content in the infected tissue may
rise up to 1%, which is sufficient to inhibit
the germination of buds (Kuo et al., 1969).
As a result, gappy stands are evident and
young crops have a patchy and uneven
appearance. In the early stages of rotting, the
disease may be diagnosed by a strong odour
of overripe pineapple. Although pineapple
disease is not considered important in standing cane, infection may occur if the stalks
are physically damaged or stressed (Wismer
and Bailey, 1989).
Causal agents
Pineapple disease is caused C. paradoxa.
The fungus belongs to Phylum: Ascomycota;
Class: Ascomycetes. The fungus produces
two types of imperfect spores, conidiospores
6–24 um × 2–5.5 µm (thin-walled cylindrical
conidia) and chlamydospores 10–25 um ×
7.5–20 µm (thick-walled and oval). These
spores are produced intensively on the
internal tissues of the infected seed pieces.
They are released into the soil on seed piece
decay. The spores may survive for several
years in the soil, serving as a source of inoculum for the next crop (Wismer and Bailey,
1989). The perfect stage of the fungus has
been reported and it occurs naturally on
cacao (Dade, 1928) and sugarcane (Kuo
et al., 1969). The pathogen also causes diseases of pineapple, banana, cacao, coconut
and oil palm (Wismer and Bailey, 1989).
Disease symptoms
The disease affects sugarcane setts in the first
weeks after planting. The fungus spreads
rapidly through the parenchyma and colonizes all the internal tissue of the seed piece,
which turns red and eventually black. The
black coloration results from the production
of fungal spores within the seed piece. Nodes
act as partial barriers to the spread of rotting,
but with susceptible varieties, entire seed
pieces may become colonized by the fungus
(Wismer and Bailey, 1989). The pineapple
odour resulting in the decayed seed pieces
is due to ethyl acetate, formed by the metabolic activity of the pathogen. The ethyl
Disease control
Control is a priority, especially where the
soil inoculum level is high through the amelioration of conditions that favour the germination of buds and the emergence of
young shoots (e.g. good quality cuttings,
adequate irrigation, right planting time and
depth). The ends of cuttings are dipped in a
fungicidal solution, either as a cold dip or
the fungicide may be incorporated into the
water tank at the time of the hot water treatment (Antoine, 1956). Other fungicides
used to control pineapple disease include
226
A.M.H. Esh
benomyl, propiconazole carbendazim, etc.
(Autrey, 1974). Systemic fungicides are found
more efficient than non-systemic fungicides
(Vijaya et al., 2007).
Since pineapple disease is a soilborne
disease, crop rotation or a fallow period
between cane crops may prove to be of some
benefit in reducing its impact. Seed-piece
infection by pineapple disease frequently
proceeds from the exposed cut ends to the
centre of the seed piece. Therefore, the use
of seed pieces containing at least three
nodes increases the likelihood that buds
closer to the centre will germinate (Wismer
and Bailey, 1989).
For disease control, sett treatment with
chemical fungicides before planting is an
effective method and is widely followed.
Systemic fungicides, benomyl and carbendazim, were found more efficient than
non-systemic fungicides such as captan and
mancozeb (Vijaya et al., 2007).
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18
New and Emerging Fungal Pathogens
Associated with Leaf Blight Symptoms on
Wheat (Triticum aestivum) in Argentina
Analía Edith Perelló
CIDEFI (Centro de Investigaciones de Fitopatología) – CONICET
(Consejo Nacional de Investigaciones Científicas y Técnicas),
Facultad de Ciencias Agrarias y Forestales de la Universidad
Nacional de La Plata, La Plata, Provincia de Buenos Aires, Argentina
Abstract
Regional surveys are being conducted at the CIDEFI to investigate the presence of wheat (Triticum
aestivum L.) pathogens on leaves and seeds across the Argentinian cropping area. During the past
5 years in the wheat cropping area of Buenos Aires Province, Entre Ríos and Santa Fe Provinces,
Argentina, several unusual diseases have been found on wheat leaves. From the symptomatic tissues,
the fungi were isolated and identified. To test pathogenicity and fulfil Koch’s postulates, inoculations
of different wheat cultivars under greenhouse conditions were carried out; disease symptoms and the
causal agents are described.
Introduction
Wheat ranks as a primary source of food and
livelihood for hundreds of millions of people
globally, especially in developing countries.
Several serious foliar diseases caused by
necrothrophic pathogens occur in this crop
in Argentina. Among them, Septoria tritici
leaf blotch and tan spot caused by Drechslera
tritici-repentis are the most important. Both
can cause serious yield and quality losses
under the right conditions. Among biotrophic
diseases, leaf rust is a very dangerous one.
Samples were obtained from different
wheat cultivars from 2001 in 13 different
locations of the main wheat-growing area of
Argentina (northern, central and eastern
regions of the Buenos Aires Province, Entre
Ríos and Santa Fé). Most symptoms were
observed on the upper leaves at growth
stage 69 (anthesis complete) and at growth
stages 80 (early dough) to 85 (soft dough)
according to the scale of Zadok et al. (1974).
Samples of 20–40 plants of each disease
area were used for late laboratory identification of leaf-spotting fungi.
The frequency of the pathogens was
different among localities and cultivars over
the past 10 years. Pyrenophora tritici-repentis
was predominant, followed by Mycosphaerella graminicola. In a total of 193 and 240
infected leaf samples collected in 2006–
2007, D. tritici-repentis was observed in 63
and 77% of leaf samples, respectively, suggesting an increasing trend in incidence
over the years. This may be explained in
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
231
232
A.E. Perelló
part due to the adoption of reduced-tillage
cropping practices. Surprisingly, a significant increase in the frequency of Alternaria
spp. isolates was observed, with 41 and 53%
of the samples harbouring tan spot in the
same period. The isolates were identified as
belonging mostly to the A. infectoria species
group. Moreover, mapping of pathogen distribution during the past years in different
agroclimatic zones of Buenos Aires Province shows that A. infectoria is a widespread
pathogen that is gaining prominence as an
emerging wheat pathogen in latter years.
Also, it was found, together with A. alternata, on almost 70% of wheat seed samples.
Other diseases were sporadic, isolated
or minor and included foliar blight or spots
caused by the fungus A. triticina, A. infectoria species group complex, Bipolaris sorokiniana, Cladosporium herbarum, Phoma
sorguina, Ascochyta hordei, Pyricularia grisea, Cephalosporium gramineum and others.
Some disease symptoms are characteristic
and obvious. Other diseases, however, may
be difficult to diagnose without microscopic
or laboratory analysis. Since different diseases require different control strategies,
their accurate diagnosis is essential.
Leaf Blight of Wheat Caused by
Alternaria triticina in Argentina
Alternaria species are perhaps the most
common fungi encountered by mycologists
working in plant pathology. As plant pathogens, over 4000 Alternaria/host associations
are recorded in the USDA Fungal Host Index
and the genus ranks 10th among nearly 2000
fungal genera listed based on the total number of host records. In Argentina, this genus
has been studied on wheat plants since the
last decade (Perelló, 2007; Perelló et al., 1992,
1996, 2002, 2003, 2005a,b, 2008; Perelló and
Sisterna, 2005, 2006). Several Alternaria species and numerous uncharacterized Alternaria taxa have been found associated with
leaf blight symptoms. Alternaria species are
some of the most prodigious producers of
toxic secondary metabolites, producing over
70 compounds of varying toxicity (Kumar
and Rao, 1979). Some of these metabolites
are powerful mycotoxins not yet characterized in Argentina.
During routine investigations across
the wheat (T. aestivum)-growing area of the
Buenos Aires Province, diseased leaf samples were collected from different wheat
cultivars. Discoloured oval lesions appear on
lower leaves. The disease progresses upwards,
lesions enlarge and coalesce to irregular, dark
blotches, often with chlorotic margins.
Severely infected seeds are discoloured and
shrivelled.
Necrotic tissue fragments were surfacesterilized and plated on potato dextrose agar
(PDA), from where Alternaria specimens
were isolated. Morphobiometrical and cultural features of the fungus were examined
on potato carrot agar (PCA). Conidia were
irregularly oval, ellipsoid conical, gradually
tapering into a beak, 15–92 × 8–35 µm, with
1–10 transverse septa and 0–5 longitudinal
septa, light brown to dark olive buff, becoming darker with age. All isolates obtained
were identified as A. triticina following the
morphological descriptions by Anahosur
(1978) and confirmed by comparison with
reference strains of CABI Bioscience (IMI
289962 and IMI 178784) kindly sent by
Dr D. Mercado (Université Catholique de
Louvain, Unité de Phytopathologie, Belgique). One of the isolates has been lodged in
the culture collection of La Plata Spegazzini
(LPSC) (accession number 798). Pathogenicity tests were conducted in the greenhouse.
Susceptible wheat cultivars were inoculated
at tillering and heading stages with a conidial suspension (2 × 105 conidia/ml). After
10 days, typical leaf blight symptoms developed and A. triticina was recovered from
the lesions. No symptoms appeared on the
control plants.
Alternaria triticina causes significant
yield losses in wheat on the Indian subcontinent, from where it originates and has
spread throughout the world (Agarwal et al.,
1993). Although A. triticina has been detected
previously in Argentina on wheat leaves
and seeds (Perelló et al., 1992), it has probably existed as a minor pathogen for many
years without being noticed. The recent
increase in the severity of leaf blight may be
New and Emerging Fungal Pathogens
due to new cultural practices such as conservation tillage, nitrogen fertilization, irrigation, use of new germplasm, as well as
favourable weather conditions. As A. triticina
is a quarantine pathogen in many countries,
it is important to investigate the incidence
and importance of this disease in Argentinean wheat areas. This is the first published
record of A. triticina on wheat in Argentina
and on any host in this country.
Alternaria leaf blight
Alternaria triticina Prasada & Prabhu causes
significant yield losses in wheat in the
Indian subcontinent, from where it originated and has spread throughout the world.
The disease was first reported in 1924 but
remained incompletely characterized for
several years. Early studies associated Alternaria spp. with the disease, but the causal
organism, A. triticina, was not identified until
1962 (Prasada and Prabhu, 1962). From
1960 to 1964, leaf blight damaged all commercial cultivars on the Indian subcontinent (Prabhu and Prakash, 1973; Bhowmik,
1974; Sokhi, 1974). It developed on plants
approaching maturity, caused premature
death of the uppermost leaves and heads and
reduced yield significantly. Today, durum
wheats, their derivatives and introduced
Mexican wheats are considered most susceptible (Frisullo, 1982). In addition to wheat,
the disease affects triticale in India and other
graminaceous hosts in the Middle East and
Nigeria (Chaudhuri et al., 1976).
Symptoms
The pathogen may infect all foliar parts.
Alternaria leaf blight is characterized by
small, chlorotic, oval- or elliptical-shaped
lesions scattered on lower leaves. As the plant
matures, the disease progresses upwards and
lesions darken to brown-grey, enlarge and
coalesce to irregular, dark blotches, irregular in shape and may have a yellow margin.
The chlorotic borders of the lesions may
become diffuse and turn light to dark brown
233
in colour. The lesions develop progressively
from lower to upper leaves and blighting
may extend to heads and leaf sheaths. Symptoms appear on leaves, seeds and spikelets.
Severely infected seeds are discoloured and
shrivelled (Rault et al., 1983). Under humid
conditions, the lesions support visible clusters of dark, powdery conidia. Lesions are
difficult to distinguish from those caused by
Helminthosporium spp. Alternaria leaf blight
is likely to develop near irrigation ditches,
in low areas, or wherever humidity and soil
moisture are high. It develops rapidly once
wheat plants are 6–8 weeks old, and especially as the crop approaches maturity.
Bread and durum wheat, barley and triticale
are the primary hosts.
Causal organism
Isolations from leaf lesions routinely yield
Alternaria spp., many of which are saprophytic and mask the pathogen. Alternaria
triticina is distinguished by its wheat-specific
virulence and cultural characters. The
mycelium and conidia of A. triticina are initially hyaline and later olive buff. Conidiophores are septate, usually unbranched but
occasionally branched, erect, single or fasciculate, emerging through stomata, geniculate, straight, length variable, between septa
17–28 µm, 3–6 µm wide. A chromogenic
variant in A. triticina was studied (Jain and
Prabhu, 1976). Conidia of A. triticina are
acrogenous, borne singly or in short chains
(two to four spores). They vary from 8 to
35 µm in width and 15 to 92 µm in length, are
dark, ellipsoid to conical, tapering to a beak.
A. triticina grows on a variety of simple
media (Rao and Subrahmanyam, 1974). Colonies on PDA are discrete or effuse, dark
blackish brown to black, margin smooth
and entire. Growth is optimal between 20°
and 24°C, with limits near 5° and 35°C.
Physiological specialization of the pathogen
exists and six races have been characterized
and reported using 15 differentials. Nonspecific phytotoxins produced by the
pathogen apparently play a role in wheat
pathogenesis.
234
A.E. Perelló
Alternaria infectoria Complex
Associated with Black Point and
Leaf Blight Symptoms in Argentina
Among the field fungi found in cereals, Alternaria is the dominant genus and, within this
habitat, taxa of the A. infectoria species group
predominate by far. Information available
on the A. infectoria species group is limited
as the taxa it comprises have often been misidentified as other small-spored Alternaria
species, due to the use of insufficient methods for identification. Members of the A.
infectoria species group are morphologically
distinguishable from other small-spored
species of Alternaria by their long secondary
conidiophores and formation of white or
grey colonies on dichloran rose bengal yeast
extract sucrose media (DRYES) (Andersen
et al., 2002).
Furthermore, this species group is the
only one among Alternaria where the teleomorph, Lewia, has been identified in Argentina. To date, the A. infectoria species group
comprises the known species A. arbusti, A.
conjuncta, A. infectoria, A. oregonensi, A. triticimaculans, A. metachromatica, A. viburni,
A. intercepta and A. novae-zelandiae, as well
as an unknown number of distinct taxa yet
to be described. Members of the A. infectoria species group produce a range of unique
secondary metabolites that are useful for
metabolic profiling and chemotaxonomy of
Alternaria. Until now, four different profiles
have been identified, suggesting that there is a
potential risk of Alternaria mycotoxins in
wheat in Argentina (Pich et al., 2007).
Black point and leaf blight caused by A.
infectoria species group complex is a new
disease of wheat in Argentina. Four hundred and ten isolates collected from 17 different geographical zones of the Argentinian
cropping area were tested for their morphological variation. The isolates differed in
their morphobiometrical and cultural characteristics on PCA. On this basis, they were
categorized into four morphotypes. Further,
20 isolates were characterized according to
their pathogenic and biochemical variability. Pathogenicity tests were conducted under
greenhouse conditions on wheat plants of
the cultivars Buck Arriero, Buck Charrúa,
Buck Granar, Buck Poncho, Buck Yatasto,
Klein Cacique, K. Estrella, ProINTA Cinco
Cerros, ProINTA Elite and Pro INTA. Significant differences between cultivars, isolates and the interaction isolate × cultivar
were shown according the ANOVA results.
Analysis of severity means (Tukey’s test)
showed cultivar Buck Charrúa as the one
with the best behaviour against all the isolates tested, and cvs. Pro INTA Cinco Cerros
and Pro INTA Elite as the most susceptible.
Symptoms observed were: chlorosis and/or
apical or general necrosis (blight), or elongated necrotic spots surrounded by a chlorotic halo. Additionally, 20 samples of
wheat seeds from different localities in the
Buenos Aires Province were analysed by
blotter test (ISTA) (Neergaard, 1979). After
7 days incubation (20 ± 2°C and cycles of
12 h light plus NUV light), the microorganisms developed were identified and the A.
infectoria complex in particular was characterized according to its morphobiometrical
features on PCA. A prevalence (samples
infected/samples analysed) of 55% and
infection values of 37% of A. infectoria species group members was registered.
Twenty isolates were tested in a comparative analysis of five isoenzyme patterns (phosphatase, peroxidase, A-esterase,
B-esterase, glutaminotransaminase) and total
proteins. Mycelium for electrophoresis in
polyacrylamide gel was obtained from monosporical cultures of the fungus on PCA over
10 days. The results obtained revealed differences between the strains in the isozyme
banding patterns. Each isolate had a characteristic electromorph, showing different
main bands of enzyme activity and some
minor bands varying in intensity for all the
patterns assayed. Isozyme data corroborated
the morphological and pathogenic variability observed previously on A. infectoria isolates collected from wheat. These results
support the usage of isoenzymatic patterns
for the characterization of isolates of A. infectoria complex associated with black point
and leaf blight symptoms on wheat, as a
valuable additional tool to aid the traditional taxonomy, base on morphocultural
characters only.
New and Emerging Fungal Pathogens
Detection of Lewia infectoria
and its Alternaria Anamorph
from Wheat in Argentina
Occurrence of L. infectoria (Fuckel) Barr &
Simmons (teleomorph of A. infectoria) developed in culture is described, illustrated and
reported for the first time. Monosporic isolates, obtained from infected wheat plants,
produced conidia within a week and ascomata with fully mature ascospores within
7 months when stored on slants of PCA at 4
degrees in darkness. The anamorph exhibited the sporulation pattern of A. infectoria
species group and was identified on the
basis of axenic colony morphology and by
the prominence of their secondary conidiophore structure. Critical examination of the
teleomorph proved it to be L. infectoria. The
importance in interpreting the teleomorph–
anamorph pair is discussed.
A. infectoria species group, causing leaf
blight and black point of wheat, was not significant for many years, but currently this
constraint has become a new problem (Perelló and Sisterna, 2006). During 2005, wheat
samples were collected and typical symptoms of tan–dark brown leaf spot were
observed on several cultivars from the cropping area of Buenos Aires Province. Different
isolates of a fungus with characteristics of
Alternaria were obtained from this material.
In laboratory conditions on PCA, cultures of
this fungus produced conidia within a week.
Then, these isolates were stored to maintain
a fungal collection. Within 7 months, ascomata with fully mature ascospores of a previous undescribed genus were observed in
connection with this anamorphic state.
Based on morphological characters (Simmons, 2002), the teleomorph proved to be a
Lewia species, described here as L. infectoria
with its Alternaria anamorph.
Wheat leaves exhibiting necrotic symptoms were collected during September and
October in 2005 from different cultivars of
farmers’ fields and research stations of eight
localities of Buenos Aires. Lesions on leaves
were viewed through a stereoscope at × 12
and specific morphological characteristics
of pathogens were recorded. Fungi were cultured on PDA. After screening of the cultures,
235
180 belonged to the Alternaria genus. Single
spore cultures were obtained on PCA. Based
on morphological characters like the conidial sporulation pattern and the prominence
of their secondary conidiophore structure,
most of the strains were identified as members of the A. infectoria species group. The
cultures were stored on slants of PCA at 4°C
in darkness. Numerous conidia appeared on
the surface of the agar within a week and
groups of fruiting bodies within 7 months.
To determine their stage of development,
the fungal fruiting bodies were placed on a
microscope slide, stained with 0.25% Trypan blue in lactid acid:glycerol:water (1:1:1)
and examined with a light microscope
(× 400). After 7 months, groups of fertile
ascomata (pseudothecia) with septate hyaline mature ascospores developed in isolates
obtained from the cultivar Klein Estrella,
from a particular field in the locality of Balcarce (Buenos Aires Province). There were
detectable differences between isolates with
regard to pseudothecial density and speed of
ascospore maturity. In some cases, the production of immature asci was observed.
The morphobiometrical and cultural
features of these ascomata on PCA allowed
the identification of the teleomorph of A.
infectoria, L. infectoria (Fuckel) Barr & Simmons. Its description is as follows: ascomata
ellipsoid, 400–500 × 150 µm, with a short,
papillate beak, dark, thin-walled at maturity.
Asci 105–125 × 13–16 µm, subcylindrical,
straight or somewhat curved. Ascospores 8,
18–22 × 7–8 µm at full development, broadly
elliptic, muriform, becoming 5-septate (3 primary septa), only end cells not longitudinally septated, constricted, yellow-brown.
Although most Alternaria species do
not have teleomorphic affinities, a number
of anamorphically defined taxa within the
Pleosporaceae have recognized teleomorphs
and most are not commonly encountered
(Simmons, 1986, 2002). These teleomorphs
are representative of nearly all major lineages
within the Pleosporaceae. An evaluation of
the teleomorphic characters of well-known
Pleospora spp. with anamorphs of Alternaria,
namely P. infectoria and P. scrophularieae,
revealed that Pleospora spp. with Stemphylium anamorphs were morphologically
236
A.E. Perelló
distinct from Pleospora spp. with Alternaria
anamorphs, particularly in the size of the
ascomata and ascospores. This resulted in
the designation of the genus Lewia for
Pleospora-like fungi with Alternaria teleomorphs (Simmons, 1986). Evidence of production of Alternaria-related teleomorphs
in axenic culture was previously reported
by Bilgrami (1974) in L. infectoria, Simmons
(1986) in L. photistica, Kwasna and Kosiak
(2003) in L. avenicola and Kwasna et al.
(2006) in L. hordeicola. Other described
Lewia species, like L. chlamidosporiformans,
L. ethzedia, L. intercepta, L. sauropi, L. viburni
and L. eureka, usually produce ascomata on
tissues of infected plants (Simmons, 1986,
2002; Vieira and Barreto, 2005). During the
present study, ascomata of L. infectoria were
produced in vitro, in axenic culture on PCA
slants, in connection with the anamorph.
However, the fungus does not often
produce both anamorph and teleomorph on
the same slant. Crossing between isolates is
evidently not necessary for production of the
teleomorph since single-ascospore axenic
cultures continued to produce ascomata on
PCA. These results were similar to observations made by Kwasna and Kosiak (2003) for
L. avenicola. In addition, its finding in Argentina has provided an important framework
for hypothesis testing in advanced studies
on Alternaria/Lewia epidemiology and pathogenicity variability on wheat plants.
Lewia infectoria forms pseudothecia on
wheat straw in the field under determined
weather conditions. This could play an
important role as a source of inoculum in
Alternaria/Lewia disease able to infect wheat
and wild grasses as a result of the dispersal
of airborne ascospores. The discovery of the
sexual stage in nature may have a large
influence on localized development of the
diseases in different regions of the country.
Occurrence of Ascochyta hordei
Hara var. europaea Punith.
on Wheat Leaves in Argentina
Ascochyta leaf spot is often overlooked in
association with other leaf spot diseases. It
is reported on wheat as a pathogen of minor
economic importance but there are some
reports pointing out that high humidity conditions could favour the occurrence of outbreaks of the disease (Scharen and Krupinsky,
1971).
During September–October 2002, leaf
spot symptoms on wheat cultivar Baguette
10, growing in farmers’ fields in Tandil,
eastern area of Buenos Aires Province, were
commonly observed. The leaves showed
symptoms similar to those described for
other necrotrophic foliar pathogens (D. triticirepentis) and Stagonospora nodorum, suggesting that any of these might have been
involved. Ascochyta was commonly isolated
from affected tissues of the samples collected. A. tritici Hori & Enj. is generally
accepted as the cause of Ascochyta leaf spot,
but A. graminicola Sacc. is cited in some
literature (Zillinsky, 1984). Punithalingam
stated the status of A. tritici was uncertain
but it might be a synonym of A. hordei (Farr
et al., 1989). Sprague and Johnson (1950)
also stated that A. tritici was close to A. hordei Hara, differing mainly in the symptoms
on barley. The identity of the culture of
Ascochyta isolate A1102 of this study was
determined as A. hordei Hara var. europaea
by experts of the Centraalbureau voor
Schimmelcultures (CBS), The Netherlands,
and deposited in the CBS culture collection
under the number 112525.
Ascochyta was not previously reported
on wheat and other grasses in Argentina. In
this sense, the first occurrence of this fungus as a member of the leaf spotting complex on wheat plants in the Argentinian
cropping area is significant. Diseased leaves
were collected, stored in paper bags and
transported to the laboratory. The pathogen
was isolated from typical necrotic symptoms. Morphobiometrical and cultural studies of the fungus were studied.
Inoculation experiments to confirm
pathogenicity were performed in the greenhouse at 15–25°C and 80% relative humidity on 16 wheat cultivars: Buck Arriero,
Buck Yatasto, Buck Poncho, Buck Charrúa,
Buck Halcón, ProInta Granar, ProInta Cinco
Cerros, Desimoni Caudillo, ProInta Imperial, ProInta Puntal, ProInta Guazú, ProInta
New and Emerging Fungal Pathogens
Colibrí, ProInta Elite, Klein Estrella, Klein
Cacique and Klein Dragón. Plants were
grown in plastic pots 12 cm in diameter (4
seeds/plot in all samples) with a standard
potting mix. Plants were inoculated when
they had reached the third expanded leaf
stage. Inoculum was prepared from 10-dayold cultures of A. hordei var. europaea (isolate A1102) growing on PDA and was
obtained by flooding each sporulating plate
with sterile distilled water and gently scraping the fungal colony with a flame-sterilized
scalpel to dislodge conidia. The conidial suspension was filtered once through a single
layer of cheesecloth and spore concentration was determined with a haemocytometer. The inoculum consisted of 12 ×
106 conidia/ml. Twenty seedlings of each
cultivar were used for the inoculation.
Leaves were sprayed to runoff with a manually operated sprayer. The inoculated plants
and controls were kept in a moist chamber
for 48 h. The first symptoms appeared 92 h
after inoculation. Between 40 and 60% were
necrotic 18 days after inoculation. In natural field infections, it was observed that
plants were affected rather severely, especially the basal leaves. Lesions at first are
distinct, chlorotic, ellipsoidal or round and
1–5 mm across. Later, they become diffused
and grey-brown internally. Pycnidia sometimes form and appear as black dots within
necrotic lesions. They are submerged in host
tissues, except for a papillate projection.
In culture on PDA, pycnidia measured
142.5–225 × 93.75–206.25 µm.
Conidia
(pycnidiospores) are straight, hyaline and
oblong, 3.75–5.60 × 15–18.70 µm, typically
with one median septum.
All wheat plants inoculated with A. hordei var. europaea in the greenhouse developed symptoms identical to those observed
on naturally infected plants in the field. The
amount of damage to seedlings was measured as per cent necrotic leaf area from the
first leaf of 15 plants per each inoculated
cultivar in comparison with controls. Cultivars Buck Arriero and Buck Poncho showed
the most conspicuous symptoms 9 days after
inoculation, with a disease severity rating of
between 8–35% of necrotic leaf area. The
rest of the cultivars showed no symptoms or
237
had low disease severity. Buck Arriero, Buck
Charrúa, Buck Halcón, Buck Poncho, Buck
Yatasto and Klein Dragón showed the most
severe symptoms of the disease (between
12–45% of the necrotic leaf area) 20 days
after inoculation. The rest of the cultivars
showed little evidence of infection with
only a 10% necrotic foliar area, except ProInta Imperial and ProInta Puntal, which
showed no evidence of infection, indicating
that the disease pressure on Buck cultivars
was much higher than on the rest of the wheat
cultivars examined. These observations are
consistent with other reports (Wiese, 1977),
which indicates that the fungus is apparently of little economic consequence as a
foliar pathogen of wheat. Inoculation studies proved that A. hordei var. europaea was
the cause of this outbreak on wheat in Argentina. New cultural practices (reduced tillage,
nitrogen fertilization, irrigation), the use of
new germplasm and favourable environmental conditions could have contributed to creating ideal conditions for the increase and
spread of inoculum, not only of A. hordei
var. europaea but of the foliar complex of
necrotrophic pathogens in general. Other
wheat cultivars may also be susceptible to
isolates of the pathogen.
Phoma sorghina (Sacc.) Boerema,
Dorenbosch & van Kesteren
in Wheat Leaves in Argentina
Phoma sorghina (Sacc.) Boerema, Dorenbosch & van Kesteren is plurivorous, ubiquitous
and common in the tropics and subtropics,
causing diseases of cereals and other Gramineae and forage crops (Punithalingham, 1985;
Manavolta and Bedendo, 1999; Kumar and
Kumar, 2000). Although P. sorghina has been
found causing leaf spots in different hosts
such as Agave americana, Gossypium hirsutum, Lycium halimifolium, Lycopersicum
esculentum, Oryza sativa, Populus nigra,
Sorghum spp. and Zea mays, the disease it
causes to aerial parts of plants is of minor
importance (White and Morgan-Jones, 1983).
It causes considerable loss of seedlings of
Macroptilum, Stylosanthes and Sorghum
238
A.E. Perelló
through pre- and post-emergence death. The
fungus has been found on or associated with
sorghum grains in the humid Argentinian
Pampa (Gonzalez et al., 1997), but there are
no previous reports of its presence on wheat
plants in Argentina and therefore the confirmation of this fungus as a foliar pathogen of
wheat in Buenos Aires Province is significant. The first occurrence of leaf spot diseases of wheat by P. sorghina was in the
Buenos Aires Province of Argentina in 2002.
The fungus was detected on samples from
two localities, Olavarría and Los Hornos, on
experimental field plots with wheat cultivars, Buck Poncho and Buck Diamante.
Diseased leaves were collected, stored
in paper bags and transported to the laboratory. The pathogen was isolated from typical necrotic lesions on PDA Petri dishes.
Fifteen plants of each of the cvs. Buck
Arriero, Buck Poncho, Klein Cacique, Klein
Estrella, ProInta Cinco Cerros, ProInta Elite,
ProInta Granar, ProInta Guazú, ProInta Imperial and ProInta Puntal were grown in plastic
pots 12 cm in diameter and containing a
potting mix of clay 21.2%, lime 56%, sand
22.8%, soil organic matter (SOM) % = 3.35;
C % = 1. Plants were inoculated at the third
expanded leaf stage and heading stage. Inoculum was prepared from 10-day-old cultures
of P. sorghina growing on PDA by flooding
each sporulating plate with sterile distilled
water and gently scraping the fungal colony
with a sterile scalpel to dislodge conidia.
The resulting suspension was filtered once
through a single layer of cheesecloth and the
spore concentration was determined with a
haemocytometer. The spore concentration in
inoculum was adjusted to 1 × 106 conidia/ml.
Control plants were sprayed with sterile distilled water. Leaves were sprayed to runoff
with a manually operated sprayer. The inoculated plants and controls were kept in a
moist chamber for 48 h and observed daily.
The first symptoms appeared 72 h after
inoculation under greenhouse conditions
and 15–50% of plants showed necrotic
lesions 10 days after inoculation. The wheat
cvs. showed different degrees of susceptibility to the pathogen. The cvs. Buck Arriero, Buck Poncho and Klein Cacique became
severely infected with up to 40% of their
leaf surface spotted. The cvs. ProInta Cinco
Cerros, ProInta Elite, ProInta Granar and
ProInta Imperial were slightly spotted, with
1–5% of their foliage covered by spots. The
cvs. Klein Estrella, ProInta Guazú and ProInta Puntal were free of spots. Elongated
necrotic yellowish to light-brown lesions
could be observed on the upper surfaces of
affected leaves of wheat cvs. Leaf spots later
coalesced to form large irregular spots with
yellow margins. Under high humidity, pycnidia developed within spots on leaves after
21 days. A gelatinous spore mass was extruded in cirri from pycnidia. No symptoms or
spots were seen on the control plants. All
wheat plants inoculated with P. sorghina in
the greenhouse developed symptoms identical to those observed on naturally infected
plants in the field. In culture, the fungus
developed dark, greyish colonies with dense
aerial mycelium with abundant, solitary or
sometimes aggregated pycnidia with characteristic beaks. Conidia were globose to
ovoid or shortly cylindrical, usually straight,
hyaline, unicellular 4–7 × 2 µm. Abundant
chlamydospores and dictyochlamydospores
were observed. Newly formed chlamydospores quickly became covered with a black
coating that obscured their brown colour.
The identification was confirmed by
the Centraalbureau voor Schimmelcultures
(CBS), Utrecht, The Netherlands. One representative isolate of P. sorghina has been
lodged in the CBS culture collection with
the accession number 112525.
Cephalosporium gramineum
Nisikado & Ikata on
Wheat Leaves in Argentina
Cephalosporium stripe is a disease of cereals that is sporadic in its distribution and
occurrence but can cause severe yield losses
when it occurs. The disease is found most
consistently in areas where frost heaving,
resulting from fluctuating winter temperatures, heavier soils and higher soil moisture
damages roots (Bruehl et al., 1976). Cephalosporium stripe is caused by Hymenula cerealis (synonym C. gramineum). This fungus
New and Emerging Fungal Pathogens
is slow growing in culture and probably in
nature, too. It produces tiny conidia on
sporodochia in the saprophytic stage on
wheat straw, but as a parasite it invades the
vascular system, where it interferes with
water movement. It is the only true vascular
parasite known to attack wheat.
Hosts
C. gramineum attacks most winter cereals,
but especially wheat. It invades several
grasses (Bromus, Dactylis, Poa) and probably was indigenous to the region in native
grasses. Until now in Argentina, it has been
detected on Bromus and wheat plants only.
Disease symptoms
Cephalosporium stripe is first observed in
the spring as distinct yellow stripes on leaf
blades, sheaths and stems. The stripes may
contain thin brown streaks (necrotic vascular tissues) surrounded by yellow. Frequently, a yellow stripe on the leaf blade
continues as a single brown line down the
leaf sheath.
Nodes are darker than normal on diseased plants and, when cut lengthwise, the
inner nodal tissue is brown in colour. Plants
are stunted and the heads are white and
sterile. If any seed is set, it is usually shrivelled. Diseased plants have a scorched
appearance when hot weather accentuates
moisture stress. The fungus survives for as
long as 4–5 years in undecomposed infested
straw.
In 2004, on a non-tilled wheat assay
sown at the Julio Hirschhorn Experimental
Station, belonging to the Facultad de Ciencias Agrarias y forestales de la Universidad
Nacional de La Plata, Argentina, chlorotic
stripes, which became necrotic, were observed on leaves of wheat cultivar Buck Biguá.
Samples were collected and remitted to the
laboratory at the CIDEFI. Morphocultural
and morphobiometrical characteristics allowed identification of the fungus as C.
graminearum. The fungus was cultured on
239
2% PDA Petri dishes. Wheat plants of the
same cultivar were inoculated with a pathogen conidial suspension by a manual sprayer
under greenhouse conditions. Similar symptoms developed from 7 to 21 days after the
inoculation.
Occurrence of Cladosporium
herbarum on Wheat Leaves
in Argentina
Cladosporium on wheat was reported to be
a common and mild parasite affecting dead
or half-dead plant tissues in association
with some other fungi. It often appears on
the ear heads, causing a greenish black
mouldy growth on the affected parts (Wiese,
1987). It was not found to cause severe symptoms on leaves and stems of wheat, but there
were some reports pointing out that moist
and shady conditions could favour the
occurrence of outbreaks of the disease on
leaves (Arya and Panwar, 1955).
During the past 5 years, leaf spot symptoms on wheat cultivars Buck Pingo, Buck
Biguá, Buck Brasil and Buck Poncho growing in the north-east of the Buenos Aires Province, were commonly observed. Infected leaf
samples were collected during September–
November in an extensive survey conducted
in 2002. Samples were collected from different cultivars in farmers’ fields and one
experimental research station across the
wheat region of Buenos Aires and Entre
Ríos Provinces, in five of the eight sites surveyed (Los Hornos, Nogoyá, Olavarría,
Tandil and Victoria). In most of the plants,
leaves showed symptoms sufficiently similar to those described for the complex of
necrotrophic foliar pathogens, i.e. D. triticirepentis (Died.) Shoem., S. tritici Rob. in
Desm., A. triticimaculans Simmons & Perelló and B. sorokiniana (Sacc.) Shoem., suggesting that any of these might have been
involved. C. herbarum was commonly isolated from affected tissues of the samples
collected and it has emerged as a widespread and serious foliar disease. The fungus was previously reported in Argentina
(Marchionatto, 1948) as C. herbarum Link.
240
A.E. Perelló
var. cerealinum Sacc. on leaves and spikes
of wheat and other grasses without describing the symptoms in detail. Since then,
there have been no other reports of this disease on wheat leaves in Argentina.
Diseased leaves were collected, stored
in paper bags and transported to the laboratory. The pathogen was isolated from typical necrotic symptoms. On PDA Petri dishes,
morphobiometrical and cultural studies of
the fungus were conducted on single spore
colonies grown in Petri dishes containing
PDA, cultured at 20 ± 2°C under cool-white
fluorescent light supplemented with near
UV with a 12 h photoperiod.
Inoculation experiments to confirm
pathogenicity were performed in the greenhouse at 15–25°C and 80% relative humidity. Fifteen plants of each of the cultivars
Buck Biguá, Buck Brasil, Buck Pingo and
Buck Poncho were grown in plastic pots
(12 cm diameter) with a standard potting
mix. Plants were inoculated when they had
reached the third expanded leaf stage and
heading stage. Inoculum was prepared from
10-day-old cultures of C. herbarum (isolates
Ch101 and Ch500) growing on PDA and was
obtained by flooding each sporulating plate
with sterile distilled water and gently scraping the fungal colony with a flame-sterilized
scalpel to dislodge conidia. The conidial suspension was filtered once through a single
layer of cheesecloth and spore concentration was determined with a haemocytometer. The inoculum consisted of 2 × 105
conidia/ml. Control plants were sprayed
with sterile distilled water only. Leaves
were sprayed to runoff with a manually
operated sprayer. The inoculated plants and
controls were kept in a moist chamber for
48 h. The plants were observed periodically. The first symptoms appeared between
72 and 92 h after inoculation. All cultivars
showed susceptibility to both of the isolates
tested. Between 12 and 75% were necrotic
10 days after inoculation. Reisolation from
leaves with lesions was performed and the
isolates were compared morphologically with
those used for inoculation to fulfil Koch’s
postulates. In natural field infections, amphigenous, irregular yellowish brown spots
were observed that especially affected the
basal leaves rather severely. They often confluenced and elongated, developing progressively from lower to upper leaves. The
margin of top leaves became brittle when
dried and the tissue tore. When lesions
spread over the leaf surface, they caused the
death of the entire leaf. A velvety olivaceous
grey mould of spores and mycelia developed on the surface of the infected tissue,
forming dense tufts. Microscopic examination revealed the presence of conidiophores
more or less erect, septate, sparsely branched;
the spores are often in chains of 2 or 3, subcylindric, pale olive, 1-(2-3) septate, 10–15 ×
4–7 µm. The teleomorph, M. tulasnei (Jancz.)
Rothers was not seen.
All wheat plants inoculated with C.
herbarum in the greenhouse developed
symptoms identical to those observed on
naturally infected plants in the field. No differences in degree of infection were noted
among the cultivars. Nevertheless, adult
plants showed more severe symptoms than
younger ones. No symptoms were observed
in the control non-inoculated plants. Isolation from symptomatic tissue has consistently yielded cultures of C. herbarum. The
fungus sporulated on the diseased tissue in
the Petri dishes. Comparison of morphological characteristics of C. herbarum isolates
revealed no differences between field- and
glasshouse-produced spores, according to
the shape and size of conidia.
The isolates of C. herbarum have been
lodged in the culture collection of the CIDEFI
(Centro de Investigaciones de Fitopatología),
Facultad de Ciencias Agrarias y Forestales
de la Universidad Nacional de La Plata,
Buenos Aires, Argentina, with the accession
numbers 111-01, 209-02, 210-02, 212-02
and 215-02.
Inoculation studies proved that C. herbarum was the cause of this outbreak on
wheat in Argentina. In the past few years,
the increased incidence of the disease may
be related to new cultural practices (reduced
tillage, nitrogen fertilization, irrigation), the
use of new germplasm and favourable
weather conditions. This contributed to a
major spread, not only of C. herbarum but
also of the foliar complex of necrotrophic
pathogens in general.
New and Emerging Fungal Pathogens
The fact that other wheat cultivars apart
from those checked may also be susceptible
to the pathogen shows the importance of
conducting thorough research to determine
the reactions of those cultivars currently
used in the Argentinian cropping area.
Pyricularia grisea on
Wheat Leaves in Argentina
During 2006/2007, P. grisea (Cooke) Sacc.
was detected for first time in the north-east
region of Argentina, a non-traditional, marginal culture area. Plants of wheat cv. Klein
Chajá presented spots or blight symptoms
on all aerial parts. Isolates and pathogenicity tests confirmed the presence of P. grisea
associated to blight symptoms on leaves,
sheets and spikles. Pyricularia grisea also
affects rice and other gramineous spontaneous species in the region. Simultaneously, during 2007, the fungus was isolated
from wheat plants cvs. Cronox, Baguette
11, ACA 304 and BioINTA from Bragado,
Baradero, Rojas, Alberti and 9 de Julio
localities from the typical wheat area in
Argentina.
Conclusion
Wheat (T. aestivum L.) cultivars currently
grown in Buenos Aires Province, Argentina,
are susceptible to different leaf spotting
fungi. Surveys conducted over several years
in Argentina have determined that the main
fungi involved in this disease complex are
M. graminicola (Fuckel) Schroet. in Cohn
(anamorph S. tritici Roberge in Desmaz.)
(leaf spot), Cochliobolus sativus (Ito &
Kuribayashi) Drechs. ex Dastur (anamorph
B. sorokiniana (Sacc.) Shoemaker (spot
blotch) and P. tritici-repentis (Died.) Drechs.
(anamorph D. tritici-repentis (Died.) Shoemaker) (tan spot). A. triticimaculans Simmons & Perelló was first described on wheat
in Argentina in 1996 and commonly observed
since then, like others members of the infectoria complex. During surveys of wheat
commercial fields from 2001 to 2002 to now
241
on the northern, central and southern prairies
of the Buenos Aires Province, a dramatic difference was observed between fungal diseases. A high incidence of D. tritici-repentis
was commonly observed in all locations
analysed.
Tan spot, caused by the fungus P. triticirepentis (Died.) Drechs. (anamorph D. triticirepentis) (Died.) Shoem., is a major disease
of wheat (T. aestivum L.) worldwide (Wiese,
1977; Hosford, 1975). The disease has a fast
growth in the Southern Cone region of South
America including Argentina, where it was
found for the first time affecting wheat crops
in the north-central region of the Buenos
Aires Province in the early 1980s (Annone,
1985). Subsequently, tan spot has gained
predominance among other wheat diseases
in most wheat-growing areas in the country
(Kohli et al., 1992; Annone, 1997; Carmona
et al., 1999; Perelló et al., 2003). Tan spot
was often observed throughout the growing
season and was the most common leaf disease observed each year, in 72.6% of all
wheat fields in 2001 and 90.4% in 2002.
Additionally, strains of Alternaria spp. from
wheat plants with symptoms of leaf blight
sufficiently similar to those described for
tan spot were collected from 11 localities.
All isolations corresponded to the A. infectoria species group (Simmons, 1994; Simmons, personal communication, 2001; Perelló
et al., 2002). Leaf blight of wheat caused by
Alternaria spp. isolates was not significant
in complex of wheat foliar diseases in
Argentina for many years but, currently,
this pathogen has become a new problem in
the Buenos Aires Province. Symptoms are
often difficult to distinguish in the field
from those caused by D. tritici-repentis
(Died.) Shoem.
Other pathogens for wheat, like C. herbarum, P. sorghina, C. gramineum, Pirycularia oryzae and A. tritici were registered for
the first time in Argentina (Perelló, 2007).
The pattern of diseases produced by these
phytopathogens in some areas of cultivation is changing drastically due, among
other causes, to new market trends that
induce changes in agricultural practices
and the introduction of new crops. Monitoring these changes is important in order to
242
A.E. Perelló
take appropriate and timely action to prevent disease dispersal. Information on the
most common leaf spotting fungi would
help to identify appropriate benchmarks for
selecting for disease resistance in different
environments. It would also help breeders
to set priorities in the incorporation of disease resistance to the leaf spotting complex
into adapted wheat cultivars.
Moreover, widespread occurrence of
these fungal diseases in the major wheatgrowing region of Argentina described warns
regional breeders and pathologists to increase
efforts to manage the spotting wheat leaf
complex in order to avoid future epidemics.
Acknowledgement
The author is grateful for the financial support of Project 11/A142 ‘Patógenos fúngicos
del trigo y su posibilidad de biocontrol con
microorganismos antagonistas en el marco de
una agricultura sustentable’ of the Programa
de Incentivos a la Docencia e Investigación de
la Universidad Nacional de la Plata.
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19
Diseases of Fenugreek
(Trigonella foenum-graecum L.) and
Their Control Measures, with Special
Emphasis on Fungal Diseases
S.N. Acharya,1 J.E. Thomas,2 R. Prasad1,2 and S.K. Basu1,2
1Agriculture
2Department
and Agri-Food Canada Research Centre, Lethbridge, Canada;
of Biological Sciences, University of Lethbridge, Lethbridge, Canada
Abstract
Fenugreek (Trigonella foenum-graecum L.) is an annual legume crop cultivated in India, the Mediterranean region, China, parts of Africa, Europe and Australia and, in recent years, in North America.
Although traditionally used as a spice crop, fenugreek has important medicinal and nutraceutical
properties and is also grown as a forage crop in some countries. This multi-use crop has the potential
to expand into new areas, as well as increase in the area where it is traditionally grown. Therefore, its
reaction to biotic and abiotic factors that can limit its production deserves special attention. Although
this review contains a discussion on all fenugreek diseases and insect pests, the main focus is on the
causal organisms, symptoms and corresponding control measures for all of the major and minor fungal
diseases affecting its productivity. It is interesting to note that only a few diseases have been reported
to affect this crop adversely. The two major fungal diseases that affect fenugreek are powdery mildew
caused by Erysiphe polygoni and Cercospora leaf spot caused by Cercospora traversiana. However,
disease problems may change as this crop is grown more widely and with larger acreages outside of its
natural area of adaptation. Ongoing vigilance in disease monitoring and development of new resistant
varieties is needed to ensure productivity and usefulness of this crop in the future.
Introduction
Fenugreek (T. foenum-graecum L.) is an
annual crop belonging to the legume family Fabaceae. Although widely cultivated
in India, China, northern and eastern
Africa, parts of Mediterranean Europe,
Argentina and Australia (Acharya et al.,
2006a), it was only recently introduced to
North America (Acharya et al., 2006b). Fenugreek is a dicotyledonous, self-pollinated
plant with trifoliate leaves, branched stems
and white flowers, which typically produces
golden yellow seeds (Basu et al., 2008). It
has two morphological forms of flowering
shoots, the common one bearing axillary
flowers and an indeterminate growth habit,
whereas plants with blind shoots possess
both axillary and terminal flowers with a
more determinate growth habit (Busbice
et al., 1972; Fehr, 1993; Acharya et al., 2008;
Basu et al., 2008). Although both closed and
open type flowers are reported, the majority
of fenugreek flowers belong to the closed
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
245
246
S.N. Acharya et al.
type (Petropoulous, 2002; Acharya et al.,
2008).
Fenugreek is capable of fixing atmospheric nitrogen in the soil. The plants require
a minimal amount of nitrogen for growth,
reducing the need for nitrogen fertilizers to
supplement crop growth and facilitating
use of the plant in crop rotations. Fenugreek
is considered a dryland crop; water requirements are low, making cultivation of fenugreek an increasingly attractive alternative
to producers in regions with limited water
supply. Use of fenugreek in arid and semiarid environments can reduce the cost of
irrigation, reduce the potential for eutrophication of surface water and limit contamination of groundwater sources (Basu et al.,
2004; Acharya et al., 2008). According to
Acharya et al. (2004), dryland adaptation of
fenugreek was a major consideration for
introducing it as a forage crop for growth in
the temperate climates of western Canada.
Well-drained, loamy soils are most favourable for the crop (Rosengarten, 1969; Acharya et al., 2008), while heavy and wet soils
are known to restrict fenugreek growth
(Petropoulos, 1973; Acharya et al., 2008)
and increase susceptibility of the plant to
disease.
Although fenugreek is known primarily
as a spice crop used especially in India and
Mediterranean regions for cooking, the species name foenum-graecum refers to ‘Greek
hay’, highlighting its use as a forage crop in
early years (Acharya et al., 2006b). Fenugreek also has been referred to as a medicinal herb, both in Indian Ayurvedic and
traditional Chinese medicines (Tiran, 2003).
Its leaves and seeds have been used extensively to prepare extracts and powders for
medicinal use like wound healing and promotion of lactation in weaning mothers
(Basch et al., 2003; Tiran, 2003; Acharya
et al., 2007a). The medicinal value of fenugreek comes mainly from three chemical
constituents; i.e. steroidal sapogenins, galactomannans and isoleucine (Acharya et al.,
2006a, 2007a, 2008). Steroidal sapogenins
are often used as a raw precursor for the
production of steroidal drugs and hormones
such as testosterone, glucocorticoids and
progesterone (Fazli and Hardman, 1968) and
are also effective in the treatment of hypercholesterolaemia (McAnuff et al., 2002).
Fenugreek galactomannans appear to aid in
the control of type 2 diabetes in both animals (Raju et al., 2001; Tayyaba et al., 2001;
Puri et al., 2002; Vats et al., 2002, 2003) and
humans (Sharma et al., 1996; Puri et al.,
2002). The amino acid isoleucine is a precursor of 4-hydroxyisoleucine, which is known
to regulate the secretion of insulin in animals
(Broca et al., 2000) and also making it potentially useful in the control of diabetes.
Since fenugreek can be used for multiple purposes (e.g. as a spice, forage crop,
eco-friendly dryland crop and for medicinal
and nutraceutical applications), there is
interest in cultivation of the plant in new
biogeographical areas of the world (Acharya
et al., 2008; Basu et al., 2008). Acharya et al.
(2006b) have described fenugreek as a traditional ‘Old World’ crop with significant
potential for use in the ‘New World’. ‘Tristar’
is the first North American variety of fenugreek released by a research group in Canada (Acharya et al., 2007c). As a result of its
increased economic and industrial importance, those involved in fenugreek production need to become more aware of the
diseases that can affect both yield and quality
of the plant adversely. In many fenugreekgrowing areas, infectious and non-infectious
diseases are becoming an important production constraint because of their ability to
cause variation in crop yield and quality
(Basu et al., 2006a). When fenugreek is
introduced to a new biogeographical area,
new diseases may emerge that can cause a
reduction in productivity, and even crop
losses (McCormick and Hollaway, 1999;
Fogg et al., 2000). This review looks at major
and minor diseases of fenugreek reported
worldwide, as well as emerging fungal diseases that are increasing in importance, to
use of the plant commercially.
Diseases of Fenugreek
Fenugreek production is affected by both
biotic and abiotic agents. Abiotic diseases
or disorders are non-infectious and are often
Diseases of Fenugreek
caused by a deficiency in nutrients, extremes
in temperature, moisture, soil acidity or
alkalinity, an excess of certain micronutrients within the soil and toxic impurities
in the atmosphere (Petropoulos, 2002;
Acharya et al., 2008). For example, Sinskaya (1961) reported yellowing of some
fenugreek plants under field conditions
due to mineral deficiencies in boron, magnesium, manganese or potassium. Physiological diseases resulting from abiotic
agents can lead to premature death of the
plant and loss of forage and seed yield. In
western Canada, exposure of fenugreek
crops to very dry and hot conditions has
resulted in stunted growth and yellowing,
with occasional loss of leaves from the
plant.
Diseases caused by living or biotic
agents (pathogens) are often infectious (Acharya et al., 2008). The most important diseases
of fenugreek are caused by plant pathogenic
fungi. Bacterial diseases are next in degree
of importance, followed by viral diseases
(AAFRD, 1998; Fogg et al., 2000; Prakash
and Sharma, 2000; Petropoulos, 2002; Weiss,
2002; Jongebloed, 2004). While information
on specific pests and diseases damaging fenugreek is limited, in general, insects and
pathogenic organisms that attack other common legume crops grown in the vicinity of
fenugreek, such as alfalfa, can also attack
fenugreek (Basu et al., 2006b).
Viral diseases
Bean yellow mosaic virus, alfalfa mosaic
virus, cowpea mosaic virus, soybean mosaic
virus, pea mosaic virus, potato virus A and
Y and clover vein mosaic virus are all common viral infections of fenugreek (Petropoulos, 2002). Bhasker and Summanwar
(1982) reported mosaic wilt on fenugreek.
Flexuous rod-shaped viruses like bean yellow mosaic potyvirus (Singh, 1969) and pea
streak carlavirus (Hagedorn and Walker,
1949) have also been reported on fenugreek.
These viral diseases have been associated
with moderate losses of fenugreek seed and
forage yield (Table 19.1).
247
Bacterial diseases
McCormick and Hollaway (1999) found that
infection of fenugreek with Pseudomonas
syringae resulted in bacterial blight. First
reported in Victoria, Australia, infection
with these bacteria caused small isolated
patches, to entire crop loss in the field. Fogg
et al. (2000) reported the same disease on
fenugreek in New Jersey, USA. It has also
been suggested that the bacterium Xanthomonas alfalfa can infect fenugreek (Petropoulos, 2002), leading to loss in productivity
(Table 19.1).
Nematode diseases
Various nematodes, typically not identified
as a problem for other crops, can damage
fenugreek roots (Weiss, 2002; Jongebloed,
2004). The soilborne nematode Meloidogyne incognita has been shown to cause root
rot and the death of immature fenugreek
plants in Australia (Jongebloed, 2004)
(Table 19.1). However, it is interesting to
note that fenugreek has also been reported
to have some anti-nematicidal properties.
Zia et al. (2003) reported that decomposed
seeds of fenugreek caused a marked reduction in population densities of the soil nematode M. javanica, which causes root-knot
development in mungbean. Decomposed
seed and aqueous extracts of fenugreek were
also able to enhance plant height and shoot
fresh weight in mungbean.
Insect pests
In Australia, insects such as thrips, podborers and Heliothis can cause serious damage to forage yield in fenugreek (Lucy, 2004).
Basu et al. (2006b) reported that in southern
Alberta (Canada), a low level of insect pests
such as Lygus bugs and, to a lesser extent,
alfalfa plant bugs and aphids had been
observed in fenugreek fields. In addition,
the researchers reported that western flower
thrips (especially severe under greenhouse
conditions), alfalfa looper, alfalfa weevil
248
S.N. Acharya et al.
Table 19.1. The major non-fungal diseases of fenugreek reported worldwide.
Disease
groups
Viral
diseases
Bacterial
diseases
Nematode
diseases
Insectmediated
diseases
Tolerant
varieties/
genotypes
Causal organisms
Country
reported
Bean yellow mosaic virus
England
Potato virus A
NA*
Fluorescent,
Ethiopian
NA
Cowpea mosaic virus
NA
NA
Potato virus Y
Tobacco etch virus
Pea streak virus
NA
NA
NA
NA
NA
NA
Pea mosaic virus
Soybean mosaic virus
NA
NA
NA
NA
Alfalfa mosaic virus
NA
NA
Tomato black ring virus
NA
NA
Clover vein mosaic virus
NA
NA
Pseudomonas syringae
pv. syringae
Xanthomonas alfalfa
Meloidogyne incognita
Australia
NA
Australia
NA
NA
Brunt, 1972; Petropoulos,
1973, 2002
Schmelzer, 1967;
Anonymous, 1968
Vidamo and Conti, 1965;
Anonymous, 1968
Schmelzer, 1967
Petropoulos, 2002
Hagedorn and Walker,
1949; Anonymous, 1968
Petropoulos, 2002
Quantz, 1968; Schmelzer
and Wolf, 1971
Quantz, 1968; Schmelzer
and Wolf, 1971; Latham
and Jones, 2001
Quantz, 1968; Schmelzer
and Wolf, 1971
Quantz, 1968; Schmelzer
and Wolf, 1971
McCormick and Hollaway,
1999; Fogg et al., 2000
Petropoulos, 2002
Jongebloed, 2004
Lygus keltoni, L. elisus,
L. borealis and L. lineolaris
Adelphocoris lineolatus
Acyrthosiphon pisum
Frankliniella occidentalis
Sitona sp.
Hypera postica
Autographa californica
Aphis craccivora
Canada
Tristar
Basu et al., 2006a
Canada
Canada
Canada
Canada
Canada
Canada
India,
West Asian
countries
India,
West Asian
countries
Australia,
India, the
Mediterranean
region
India
Sudan
India
India
India
India
Tristar
Tristar
Tristar
Tristar
Tristar
Tristar
NA
Basu et al., 2006a
Basu et al., 2006a
Basu et al., 2006a
Basu et al., 2006a
Basu et al., 2006a
Basu et al., 2006a
Weiss, 2002
NA
Weiss, 2002
NA
Weiss, 2002; Lucy 2004
NA
NA
NA
NA
NA
NA
Weiss, 2002
Weiss, 2002
Weiss, 2002
Weiss, 2002
Weiss, 2002
Weiss, 2002
Myzodes persicae
Scirtothrips dorsalis
Tetranychus cucurbitae
Pachymerus pallidus
Diacrisia oblique
D. orichalcea
Prodenia litura
Maruca testulalis
Note: *NA = not available.
References
Diseases of Fenugreek
and Sitona sp., were attracted to standing
fenugreek crops under field conditions in
western Canada. Aphis craccivora and
Myzodes persicae have caused damage to
fenugreek crops from west Asia to India,
while various Thysanoptera (thrips), including Scirtothrips dorsalis, have been found
on almost all fenugreek crops grown from
the Mediterranean to India (Petropoulos,
2002; Weiss, 2002). There have also been
reports of mite (Tetranychus cucurbitae)
attacks on fenugreek in India (Weiss, 2002).
Pachymerus pallidus, a seed beetle, which
attacks a wide range of crops, is a major pest
of fenugreek in the Sudan (Weiss, 2002). A
number of polyphagous caterpillars belonging to the order Lepidoptera, including Diacrisia oblique, D. orichalcea and Prodenia
litura, and especially the mung moth (Maruca
testulalis), have been reported to affect fenugreek in India (Weiss, 2002) (Table 19.1).
Common Fungal Diseases
of Fenugreek
The two most common fungal diseases infecting fenugreek are Cercospora leaf spot and
powdery mildew (AAFRD, 1998). Powdery
mildew on fenugreek, caused by E. polygoni,
can seriously reduce crop yield (Prakash
and Sharma, 2000; Jongebloed, 2004) and
has the potential to affect biomass and seed
yield in crops grown under moist agroclimatic conditions in North America. In Australia, yield of fenugreek was seriously
affected by blight caused by C. traversiana
and wilt caused by Fusarium oxysporum
and Rhizoctonia solani (Jongebloed, 2004).
The pathogen C. traversiana is spread by
contaminated seed and is now found in many
countries, ranging from India to Europe, eastern Africa including Ethiopia and in several
countries in South America; it is slowly
becoming a major fenugreek disease concern
(Weiss, 2002). Other well-known fungal diseases observed to be associated with fenugreek are collar rot, leaf spot and pod spot
diseases (Petropoulos, 2002) (Table 19.2).
In India, 27 species of fungi have been
isolated from fenugreek seeds (Prabha and
249
Bohra, 1999). Time of sowing can influence
the damage caused by an infection. For
example, in Haryana, India, seed sown in
mid October as compared with the end of
November exhibited a 30% reduction in
damage caused by E. polygoni and Leveillula taurica (Sharma, 1999). Downy mildew
caused by Peronospora trifoliorum and
spring black stem and leaf spot caused by
Phoma pinodella have recently become more
common (Lakra, 2002, 2003; Bretag and Cunnington, 2005). Several leaf diseases causing
varying degrees of damage generally or in
specific seasons, including rust due to
Uromyces anthyllidis, have been reported
in India (Weiss, 2002). Root and collar rots
caused by Rhizoctonia spp., typically R.
solani and Alternaria spp., often A. alternata, can damage individual crops (Weiss,
2002).
Antifungal activity for fenugreek has
also been reported in the primary literature (El-Gizawy et al., 2000). Lupin and
fenugreek seed extracts significantly suppressed Pythium damping-off of cucumber
and tomato seedlings, as well as radish
damping-off caused by R. solani. Moreover,
application of seed extracts had a significant
positive effect on seedling growth of the vegetables tested (El-Gizawy et al., 2000). A
detailed description of major and minor
fungal diseases of fenugreek reported all
across the globe and their prescribed control measures are outlined individually in
the following sections.
Cercospora leaf spot
Cercospora leaf spot is a seedborne fungal
disease, considered to be one of the most
serious threats to fenugreek. This disease is
capable of causing considerable economic
loss (Leppik, 1959, 1960; Khare et al., 1981;
Zimmer, 1984; Ryley, 1989). The Cercospora
leaf spot of fenugreek has been reported all
across the world and is most common in
Australia, several eastern European countries, South America, North America, in the
Near East and India (Voros and Nagy, 1972;
Cook, 1978; Khare et al., 1981; Ryley, 1989).
250
S.N. Acharya et al.
Table 19.2. The major fungal diseases of fenugreek reported worldwide.
Name of the
disease
Pathogenic fungal
species
Country
reported
Cercospora
leaf spot
Cercospora traversiana
India,
NA
Australia,
Canada
Collar rot
Rhizoctonia solani
India
Leaf spot
Ascochyta sp.
UK
Powdery mildew
Oidiopsis sp.
Downy mildew
Peronospora trigonellae
Israel,
Ethiopia,
England
India
Powdery mildew
Leaf spot
Leveillula taurica
Pseudoperiza
medicaginis
Phoma pinodella
Israel
Tolerant varieties/
genotypes
References
Leppik, 1959, 1960;
Khare et al., 1981;
Zimmer, 1984;
Ryley, 1989
TG-18, UM-20,
Hiremath et al., 1976;
Pusa Early
Hiremath and
Bunching
Prasad, 1985; Raian
et al., 1991; Petropoulos, 2002; Datta
and Chatterjee, 2004
Fluorescent,
Walker, 1952;
Ethiopian
Petropoulos, 1973
Fluorescent
Palti, 1959; Rouk and
Mangesha, 1963;
Petropoulos, 1973
HM-346, HM-350, Lakra, 2002, 2003;
HM-444
HAU, 2008
NA*
Palti, 1959
NA
Glaeser, 1961
Australia
NA
Erysiphe polygoni
Israel,
Ethiopia,
India,
Canada
HM-350,
HM-444,
Fluorescent
Rust
Pod spot
Uromyces trigonellae
Heterosporium sp.
Israel
UK
Charcoal rot
Macrophomina
phaseolina
Sclerotinia trifoliorum
Fusarium oxysporum
Pakistan
NA
Kenyan,
Moroccan
NA
Spring black stem
and leaf spot
Powdery mildew
Root rot
Fusarium wilt
UK
India,
Sudan,
Malta
NA
NA
Bretag and
Cunnington, 2005
Petropoulos, 1973;
Zimmer, 1984;
Prakash and
Saharan, 2000;
Basu et al., 2006a;
HAU, 2008
Ubrizsy, 1965
Petropoulos, 1973
Haque and
Ghaffar, 1992
Petri, 1934
Borg, 1936;
Komaraiah and
Reddy, 1986;
Hashmi and Thrane,
1990; Bansal and
Gupta, 2000
Note: *NA = not available.
The causal organism for this disease is C.
traversiana, a member of the Ascomycetes
(Agrios, 1997). Several researchers have
suggested that C. traversiana is the only species of the Cercospora infecting fenugreek
(Cook, 1978; Ryley, 1989). Conidiophores of
C. traversiana are dark, paler towards the tip,
unbranched, rarely geniculate and rarely
septate. These conidiophores develop in fascicles of 3–5 conidiophores per fascicle, with
a length of up to 420 µm and width ranging
from 3 to 5 µm (Ryley, 1989). The conidia are
Diseases of Fenugreek
hyaline, acicular, straight or slightly curved,
apex rounded, base truncate and multicellular. The main source of overwintering inocula
is plant debris, where sclerotia or stromata
can form. Conidia germinate best at a high
relative humidity and at a high temperature.
They are dispersed mainly by rain-splash
and to some extent by wind (Agrios, 1997).
Cercospora leaf spot initially presents
itself as circular, sunken lesions that appear
bleached in colour, with narrow (1–2 mm)
chlorotic halos on the surface of the leaves.
These lesions expand rapidly as the infection progresses, producing necrotic areas.
Each area of infection is sharply defined,
with most lesions surrounded by a characteristic yellowish halo. Lesion size is increased
significantly on mature leaves, where sporulation becomes evident, giving the lesions a
whitish, velvet-like appearance (Zimmer,
1984). Severely infected plants are reported
to have only a few leaves situated towards
Lesions surrounded by yellowish
halo and lesion size increases
considerably: advanced symptoms
251
the apex of the plant (Agrios, 1997). Stem
and pods also can become infected. Disease
symptoms on pods include discoloured
infected areas, as well as severely infected
areas that can become shrunken and twisted
(Zimmer, 1984). The life cycle of the pathogen is shown in Fig. 19.1.
Control measures
As the pathogen is often seedborne, seed treatment before planting has been an effective
control measure in some cases (Leppik, 1960;
Khare et al., 1981). However, selection of
healthy seeds as planting material may also
provide an effective control (Cook, 1978).
Rotation with crops outside of the host
range for the fungal pathogen C. traversiana
may also be useful. It appears that prevention of seed contamination by treating plants
when the pathogen is first detected will
likely be the best approach to limiting spread
Mycelia giving rise to conidiophores
and conidia
Lesions spread as infection advances
Conidia disseminated by rain-splash
and wind
Appearance of circular, sunken
bleached lesions on leaves: initial
symptom
Infected seed giving rise to an
infected plant
Life cycle of Cercospora leaf
spot on fenugreek host plant
Conidia infecting healthy plants
and healthy leaf tissue
Fungus overwintering in plant debris
may produce sclerotia or stromata
Fungus overwintering in
non-treated seeds
Fig. 19.1. The life cycle of Cercospora traversiana on fenugreek host plant.
252
S.N. Acharya et al.
of this pathogen. Spraying the plants with
fungicides such as benomyl, chlorothanolin,
Bordeaux mixture, mancozeb and maneb has
been suggested as an effective chemical
control measure (Agrios, 1997).
Collar rot
Collar rot is another important fungal disease of fenugreek and has been reported in
all parts of India (Hiremath et al., 1976;
Hiremath and Prasad, 1985; Raian et al.,
1991). The causal organism for this disease
is a member of the Basidiomycetes. R. solani
reduces yield of fenugreek causing foot-rot
and damping-off where freshly emerged
seedlings fall over and die (Petropoulos,
2002). The vegetative mycelium of R. solani
is colourless when young but turns brown on
maturity. The mycelium consists of hyphae
partitioned into distinct individual cells by a
septum consisting of a doughnut-shaped pore
(Ogoshi, 1987; Alexopoulos et al., 1996). R.
solani survives as sclerotia in the soil and on
plant tissue, and as mycelia by colonizing soil
organic matter as a saprophyte. Sclerotia and/
or mycelia present in the soil and/or on plant
tissue germinate to produce fungal hyphae
that can attack the subsequent year’s crop
(Alexopoulos et al., 1996). The pathogen primarily attacks below-ground plant parts such
as the root system, but is also capable of infecting other parts such as green foliage, seeds
and hypocotyls. The most common symptom
of the disease is damping-off (Petropoulos,
2002). Most of the severely infected seedlings may die at pre- or post-soil emergence
stages. The infected seedlings may develop
reddish-brown cankers on roots and stems
at or near ground level (Anderson, 1982;
Adam, 1988; Agrios, 1997).
Control measures
Cultivation of resistant varieties has been
suggested as the best control measure for
the disease (Prasad and Hiremath, 1985).
According to Haque and Ghaffar (1992),
seed dressing and soil drenching with
Rhizobium meliloti, Trichoderma banatum,
T. harzianum and T. pseudokonongii can
control the infection effectively. The Gram
positive bacterium Bacillus subtilis can also
be used effectively as a biological control
agent for R. solani (Tschen and Kou, 1985;
Tschen, 1987). Prasad and Herimath (1985)
demonstrated that carbendazim could be
used as a seed and dry soil mix fungicide
and that captan also could be used to drench
the soil and kill the fungus.
Leaf spot
Leaf spot is another seedborne disease of
fenugreek that is caused by fungal pathogens of the Ascochyta sp. belonging to the
Ascomycetes (Walker, 1952; Petropoulos,
2002). The fungus attacks the leaves, stems
and pods of fenugreek, reducing both yield
and quality severely. It can survive in the
soil, on infected seed and on crop residues.
The pathogen is disseminated by both wind
and rain-splash (Agrios, 1997; Petropoulos,
2002). Irregular brown to black spots with distinct margins are detected on infected leaves.
As the disease progresses, the leaves on the
plant may die and fall off. Infected seeds have
round, dark brown lesions. Seedlings from
infected seeds start rotting from the point of
seed attachment and rotting advances towards
the stem and taproot; subsequently, the young
seedlings die (Petropoulos, 2002). Cool,
moist weather is favourable for rapid dissemination and growth of the fungus (Anonymous, 1970; Agrios, 1997).
Control measures
Cultivation of tolerant genotypes is a good
idea to avoid rapid infestation of the fungus
(Agrios, 1997). To protect fenugreek plants
from primary infection, seeds can be treated
effectively with benlate, while to prevent
secondary infection, use of a frequent foliar
spray containing benlate is also recommended (Petropoulos, 2002).
Fusarium wilt
Fusarium wilt of fenugreek is caused by the
fungus F. oxysporum, an Ascomycete that
Diseases of Fenugreek
has been reported by several investigators
across the world (El-Bazza et al., 1990; Borg,
1936; Hashmi, 1988; Bansal and Gupta,
2000; Petropoulous, 2002). The pathogen F.
oxysporum is both seed and soilborne
(Komaraiah and Reddy, 1986; Hashmi and
Thrane, 1990; Bansal and Gupta, 2000; Pierre
and Francis, 2000). The pathogen can remain
in infested soils for up to 10 years. Dissemination of the pathogen occurs through seed,
soil and infested plant parts (Pierre and
Francis, 2000). Fusarium wilt first appears as
a slight clearing in veins found on the outer
portion of younger leaves, followed by downward drooping of the mature leaves. At the
seedling stage, plants infected by F. oxysporum may wilt and die soon after the symptoms appear. In mature plants, vein clearing
and downward drooping of the leaf are often
followed by stunting, yellowing of the lower
leaves and subsequent wilting of leaves and
young stems. Marginal necrosis of the infected
leaves, rapid defoliation and finally death of
the entire plant typically follow (Agrios,
1997). Browning of the vascular tissue is
strong evidence of Fusarium wilt infestation. Furthermore, symptoms become more
apparent on mature plants during the period
between blossoming and fruit maturation
(Jones et al., 1982; Smith et al., 1988).
253
infections. Fungal spores produced within
leaf spots during the growing season are
spread by splashing rain (Petropoulos, 1973).
Symptoms of the disease become visible at
the third stage of pod development and can
be seen as dark brown to black spots on the
pods that extend to produce a dark olive,
velvet-like cover. Initially, localized spots
of infection elongate transversely to the pod
axis but with time spread over the pod surface and transform into more rounded to
oblong lesions. These spots are also visible
on the stems, but are rarely found on the
plant leaves. Petropoulos (2002) suggests
that the fungus does not enter into the seeds
as the mycelium of the fungus is not buried
deeply in the epidermis of the pod and that
contamination of fenugreek seeds by this
fungus takes place specifically during the
threshing process.
Control measures
Hot water treatment of the seeds before
planting is efficient to remove the fungus
from the seeds (Pirone et al., 1960). Resistant
cultivars tolerant to that fungus have been
suggested as the best way to restrict rapid
dissemination of the disease on a standing
crop effectively (Petropoulos, 2002).
Control measures
Some effective means of controlling F. oxysporum include disinfection of the soil and
planting of the seeds with thiram or captan,
crop rotation with non-hosts of the fungus,
or use of resistant cultivars (Singh, 2001).
Pod spot
Petropoulos (1973) first investigated and
described this disease in fenugreek and
identified Heterosporium sp., an Ascomycete, as the causal agent. Heterosporium
medicaginis is the only species of Heterosporium that has been reported to be pathogenic to legumes (Karimov, 1956). The fungus
overwinters on dead leaves. Spores spread
from old plant debris to initiate new plant
Spring black stem and leaf spot
This disease of fenugreek has been reported
in Australia by Bretag and Cunnington
(2005). These investigators also identified P.
pinodella, an Ascomycete previously known
as A. pinodella (Jones, 1927), as the causal
agent of the disease. This observation is supported by another investigation conducted
by Boerema et al. (2004). Phoma sp. has been
isolated from the seeds of fenugreek in
Egypt, India, Nepal, Pakistan, Sri Lanka,
Sudan and Syria (Hashmi, 1988), suggesting
that the organism is not new to fenugreek
crops. The pycnidia of the fungus are more
or less globose, glabrous, ovoid to ellipsoid
and usually aseptate (Punithalingam and
Gibson, 1976; Bretag and Cunnington, 2005).
Onfroy et al. (1999) reported that the length
254
S.N. Acharya et al.
of conidia could range from 7.3–9.6 µm. The
pathogen persists as pycnidia and mycelia
in plant debris. It is dispersed mainly by
splashing rain and to some extent by wind.
Numerous small, irregular-shaped, dark
brown to black leaf lesions surrounded by
small chlorotic areas often appear as disease
symptoms on the leaves, petioles and stems
of the growing plant. Elongated black lesions
may also develop on the taproot (Bretag and
Cunnington, 2005). Infected plants become
stunted with a mild chlorosis. In cases of
severe infection, most of the leaves turn completely yellow, wither and the taproot system
becomes completely girdled with sharp
lesions (Bretag and Cunnington, 2005).
Control measures
Bretag et al. (2006) suggested practising
crop rotation, destruction of infected plant
portions and chemical seed treatments to
control primary infections by the disease
efficiently. When the fungus was restricted
effectively at the primary infection level, it
did not spread to advanced stages in most
disease trials conducted (Bretag et al., 2006),
suggesting that restricting primary infection
is the key to control of the disease.
Powdery mildew
Powdery mildew is one of the most common
and serious fungal diseases of fenugreek,
(a)
Fig. 19.2.
affecting both biomass and yield (Petropoulos, 2002; Basu et al., 2006a). Powdery mildew is most commonly found in hot and
humid tropical and subtropical areas, as well
as in temperate to subtemperate regions (Palti,
1959; Rouk and Mangesha, 1963; Prakash and
Saharan, 2000; Basu et al., 2006a). On the
basis of recent observations, Basu et al. (2006a)
suggested that powdery mildew could become
a serious disease problem in North America,
where fenugreek is a recent crop introduction.
Although some investigators have reported
Oidiopsis sp. as the causal organism (Rouk
and Mangesha, 1963; Petropoulos, 1973),
the majority of investigators from across the
globe consider E. polygoni (an Ascomycete)
as the causal organism for the disease (Zimmer, 1984; Prakash and Saharan, 2000; Bretag
and Cunnington, 2005; Basu et al., 2006a).
The conidiophores of the fungus are simple
and erect and the corresponding conidia are
unicellular, hyaline in colour, ellipsoidal to
cylindrical in shape (Agrios, 1997; Nyvall,
1999; Basu et al., 2006a) (Figs 19.2 and 19.3).
The conidiophores vary in size from 32.5 to
65.6 µm × 9.7 to 13.6 µm, whereas dimensions of the conidia are 22.6–48.4 µm × 12.4–
20.8 µm (Basu et al., 2006a) (Fig. 19.2). The
pathogen survives mostly by developing
cleistothecia in diseased plant debris. They
survive in soil until the next season. Ascospores are released after the disintegration
of the wall of the asci. The ascospores first
infect the lower and older leaves in the
next season. The spores are carried by the
(b)
Light microscopy images of Erysiphe polygoni conidiophores (a) and conidia (b).
Diseases of Fenugreek
(a)
(b)
(c)
(d)
255
Fig. 19.3. Scanning electron microscopy (SEM) images of healthy fenugreek upper leaf surfaces [Top,
(a) and (b)] compared to powdery mildew (caused by Erysiphe polygoni) infected upper leaf surface
[Bottom, (c) and (d)]. Left images were magnified 500×, while the right images were magnified 1000×.
wind to new hosts. The pathogen is also
known to survive as a mycelium (Sharma,
2005).
Powdery mildew is one of the easier
diseases to identify on plants as its symptoms are quite distinctive. The disease can
be identified easily by the presence of white
to grey powdery masses or distinct circular
to ellipsoidal patches on both the upper and
lower surfaces of the leaves (Fig. 19.4), on
pods but rarely on flowers, and by the strong
odour emitted by the infected plants. During the initial stages of an infection, fungal
patches appear isolated or in scattered
patches which coalesce as the infection progresses. At first, leaves near ground level are
infected, after which the whole plant can
become covered with the fungus over a
256
S.N. Acharya et al.
Fig. 19.4. Comparison of healthy fenugreek upper leaflet surfaces (centre) with infected leaflets from
the same plant.
short period of time (Fig. 19.5). The upper
surface of the leaves typically bears more
fungal structures and spores than the lower
surface (Fig. 19.4).
Severely infected leaves become irregular in shape, dry and shrivelled, resulting in
stunted growth of the whole plant (Basu
et al., 2006a) (Figs 19.4 and 19.5). Although
Zimmer (1984) first identified powdery mildew infecting fenugreek in North America,
Basu et al. (2006a) reported the first major
in-depth investigation of powdery mildew
as a major disease of fenugreek in North
America based on trials that were conducted
at different locations and under different
physico-geographic conditions and variable
climatic factors on the west and east coast
of North America and the mid interior of
Canada. The life cycle of the pathogen is
presented in Fig. 19.6.
Control measures
Petropoulos (1973) and Avtar et al. (2003)
reported variation in the sensitivity of
fenugreek genotypes to powdery mildew;
hence, use of resistant varieties has been
strongly recommended to avoid disease infestation. Basu et al. (2006a) demonstrated that
application of tilt 250E-propiconazole or
milgo-ethrinol (28% at 2.5 ml/l) and captancaptane (50%) or benlate-benomyl (50% at
2.0 g\l) could control the disease at a satisfactory level, whereas Petropoulos (1973)
showed that spraying with dinocap (8–10
oz a.i/acre in 100 gals) could also control
the disease.
Conclusions
Fenugreek is affected mostly by seedborne
fungal diseases. From our experience, and
other reports of fenugreek disease, it is clear
that powdery mildew and Cercospora leaf
spot are the two most important diseases
currently affecting this crop. These diseases
can reduce the production and quality of
fenugreek crops significantly all across the
globe. Other minor fungal diseases of fenugreek, namely collar rot, leaf spot, Fusarium
Diseases of Fenugreek
257
Fig. 19.5. Spread of powdery mildew
infection on a fenugreek potted plant in
the greenhouse at Lethbridge Research
Center, Agriculture and Agri-Food
Canada, Lethbridge, Alberta, Canada.
wilt, pod spot, spring black stem and leaf
spot, and downy mildew, have the potential
to become major fenugreek diseases in many
areas, including subtemperate climatic zones.
Use of resistant cultivars and application of
suitable chemical agents are suggested by
most research groups as potential control
measures against infection and spread of
fungal diseases. Although certain Internet
sites do make widely optimistic claims
about effective biological control of fenugreek fungal diseases, they do not have
strong evidence from multi-location and
multi-year trials to support their claims and
so are not included in this review.
Fenugreek is being cultivated in many
new areas as it becomes more widely recognized as a multiple-use crop. Development
of new fenugreek cultivars and improvement of existing cultivars with disease
resistance using conventional plant breeding methods (Acharya et al., 2007b) and
advanced plant biotechnological approaches
(Laroche, 2007) could be a good strategy to
prevent emergence of new fungal diseases
for this crop. Susceptibility of a plant to disease is determined by the genetic relationship between the plant and the pathogen.
The relationship between genes of the host
and the pathogen can determine disease
expression in the host. Genetic resistance in
plants is considered a major form of biological control of disease and is possibly the
most cost-effective and environmentally
friendly way to control crop diseases. Resistant cultivars have been used effectively to
control diseases in many crops. However,
development of resistant cultivars takes
time and so work should continue in the
interim to find chemical and other biological control agents to protect the crop from
disease and other pest damage. It should
also be noted that disease control measures
should not only be cost-effective but also
need to be environment friendly and socially
acceptable.
258
S.N. Acharya et al.
Healthy green tissue infected,
most prominent symptoms
on leaves and shoots
Ascospores and conidia disseminated by air
Conidia
Asci containing ascospores
Cleistothecium
Mycelia finally
generates condiophores
bearing conidia
Infected buds giving rise to
shoots and leaves completely
covered by fungal mycelia
Production of cleistothecia on plant parts
Life cycle of powdery mildew
on fenugreek host plant
Fungus overwintering in
non-treated dormant buds
and seeds
Young plant infected
Fig. 19.6. The life cycle of Erysiphe polygoni on fenugreek host plant.
Acknowledgements
The authors express their sincere thanks to
Mr Byron Lee, Research Technician, Electron
Microscopy and Image Analysis Laboratory,
Lethbridge Research Centre (LRC), Agriculture and Agri-Food Canada (AAFC) for his
help in taking all the light microscopy and
SEM images, Mr Doug Friebel, Technician,
Forage Lab, AAFC, LRC for his help with field
trials. The authors also extend their gratitude
to the School of Graduate Studies, University
of Lethbridge, Alberta Agriculture Research
Institute (AARI) and AAFC matching grant
initiatives for graduate student assistantships
and project funding, respectively.
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20
Fungal Diseases of Oilseed Crops
and Their Management
S.S. Adiver and Kumari
Oilseeds Scheme, Main Agricultural Research Station,
University of Agricultural Sciences, Dharwad, Karnataka, India
Abstract
During the past few years, pathogens in oilseed crops have been recognized as major forces causing
economic losses, with identification of certain important ones based on their symptoms, etiology and
also ecological zones. Recent research has helped by developing new resistant varieties and other
effective management strategies. This chapter describes the causal organisms, symptoms and management of diseases of oilseed crops like castor, groundnut, safflower, sesame and sunflower. Cultural
practices for managing certain diseases have been pinpointed. Critical stages for growth of some foliar
diseases, namely rust, early and late leaf spot of groundnut, blight and mildew of sunflower, fusarial
wilt of safflower and castor, have been identified. Recommendations are given on controlling various
diseases by chemical, botanical and other effective and eco-friendly methods. Oil is an essential household commodity required for food and daily use. Certain oils are used as therapeutic agents and are in
much demand for their conversion into energy or potential biodiesel. Losses to the tune of 20% in
certain oilseed crops need our utmost attention. Various fungal diseases of groundnut, sunflower, safflower, sesame and castor are described. Disease management with fungicides and other available
methods are illustrated.
Groundnut
Groundnut, known as poor man’s almond,
contributes about 38% to the oilseed pool of
India. India is the second largest producer
of groundnut after China. The crop is subjected to attack by numerous pests and
pathogens. Among foliar fungal diseases,
early and late leaf spots, commonly called
‘Tikka’ disease, and rust are economically
important.
Early leaf spot caused by
Cercospora arachidicola Hori
The perfect stage of the fungus is Mycosphaerella arachidis. In India, losses in yield
due to leaf spots have been estimated to be
in the range of 15–59%. Besides the loss in
pod and kernel yield, the value of fodder is
also affected adversely. Lesions are subcircular in shape and measure 1 to over 10 mm.
On the upper surface of the quadrifoliate
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
263
264
S.S. Adiver and Kumari
leaves, the lesions appear dark brown, while
on the lower surface they are a lighter shade
of brown. The early leaf spot usually has a
light to dark brown centre and a yellow halo.
These are oval to elongate in shape and have
more distinct margins than the late leaf spot
lesions. The early leaf spot pathogen survives through conidia on affected plant
debris in soil, or through conidia being carried on the pod shell.
Disease management
Tolerant varieties like GPBD-4, ICGV-86590,
ICGS-44, M-335, BG-3 and M-522 can be
grown wherever early leaf spot is severe.
Intercropping pearl millet/sorghum with
groundnut (1:3) is useful in reducing the
intensity of early leaf spot. Foliar spraying
of carbendazim (0.05%) + mancozeb (0.2%),
chlorothalonil (0.2%), difenaconazole (0.1%),
tebuconazole (0.1%) or hexaconazole (0.1%)
is recommended 2–3 times at 2- to 3-week
intervals starting from the initiation of the
disease (Adiver et al., 1995).
Late leaf spot caused by Phaeoisariopsis
personata (Burk. and Curt.) Van Arx
The perfect stage of the fungus is M. berkeleyii W.A. Jenkins. Late leaf spot is more
severe in the southern and central parts of
India. Dark brown to black, circular to subcircular lesions, measuring 1–6 mm diameter
appear on the lower surface of the quadrifoliate, where most sporulation occurs. The
lesions are black in colour and fruiting
structures occur in concentric rings on the
lower leaflet surface, giving lesions a slightly
rough appearance. The ambient temperature required is between 25 and 30°C. Prolonged leaf wetness hours and high relative
humidity (> 80%) favour infection and disease development. Conidia are disseminated by the wind and insects, leading to
secondary infection.
Disease management
Deep burying of crop residues in the soil
and removal of volunteer groundnut plants
are important measures in reducing the primary source of infection. Resistant/tolerant
varieties like Girnar-1, RG-141, IGV-87160,
ICGV-86590, ICGV-86325, R-8808, GPBD-4,
Kadari-4, Co-3,4, M-335 and BG-3 can be
grown wherever late leaf spot is severe. Foliar
spraying of carbendazim (0.05%) + mancozeb
(0.2%), chlorothalonil (0.2%), difenaconazole (0.1%) or hexaconazole (0.1%) is
recommended 2–3 times at 2- to 3-week
intervals, starting from the initiation of the
disease (Adiver et al., 1995).
Rust caused by Puccinia arachidis Speg.
Rust of groundnut is prevalent throughout
India; however, it is more severe in the
southern states. In India, losses in yield due
to rust alone have been reported in the range
of 10–52%, depending on the variety. Rust
can be recognized readily as orange-coloured
pustules (uredinia) that appear on the lower
leaflet surface and rupture to expose masses
of reddish-brown urediniospores. Pustules
appear first on the lower surface and, in
highly susceptible cultivars, the original
pustules may be surrounded by colonies of
secondary pustules.
Disease management
Early sowing in the first fortnight of June is
recommended to avoid incidence. Use of
resistant/tolerant varieties like Girnar-1,
ICGV-87160, ICGV-86590, DRG-12, ALR-2,3,
Co-4, ALR-1, ICGS-5 and DRG-17 is recommended. Spraying mancozeb (0.2%), tridemefon (0.1%), chlorothalonil (0.2%),
difenaconazole (0.1%), tebuconazole (0.1%),
hexaconazole (0.1%) or cyproconazole
(0.1%) 2–3 times at 2- to 3-week intervals
starting from the initiation of the disease
helps to control the disease.
Seed and seedling diseases of groundnut
Pre-emergence seed rot and post-emergence
seedling mortality are of common occurrence. The disease develops either from the
Fungal Diseases of Oilseed Crops
fungi already present in the seed or result
from direct invasion of seeds and seedlings
by soil fungi. Among seedling diseases, collar rot, root rot and stem rot are of economic
importance and are known to reduce yields
by 25–50%.
Collar rot caused by
Aspergillus niger van Tiegh
In India, collar rot, also known as crown rot
or seedling blight, is prevalent in almost all
groundnut-growing states, causing 28–50%
losses. The diagnostic symptoms are preemergence rotting of seeds and rotting of
hypocotyls, but the most common cause of
loss is early post-emergence seedling blight.
The first symptom in emerged seedlings is
usually a rapid withering of the entire plant
or its branches. Lesions develop on the stem
below the soil and spread upwards along
the branches. The dead and dried branches
are easily detached from the disintegrated
collar region.
265
in both dry weight and oil content of groundnut kernels. The first symptom is partial or
complete wilting of the stem or branch that
is in contact with the infected soil. The
leaves turn brown and wilt, but remain
attached to the plant. The pathogen has a
wide host range. S. rolfsii can colonize
either living plant tissue or plant debris.
Deeply buried sclerotia survive a year or
less, while those near the soil surface remain
viable for many years. Disease development
occurs when soil moisture is 40–50%.
Generally, when the temperature remains
between 29 and 32°C during the day and
seldom drops below 25°C during the night,
the disease develops more favourably.
Disease management
Deep ploughing, early sowing and close
planting is recommended and rotation of
groundnut with cotton, maize, sorghum and
pearl millet. Seed treatment with T. viride/
T. harzianum at 4.0 g/kg seed or seed treatment with carbendazim/captan at 2–3 g/kg
seed is suggested.
Disease management
Avoiding deep sowing (not more than 5 cm),
mixed cropping with moth bean in alternate
rows, deep tillage and early sowing of crop
is recommended. Soil application of neem
cake/castor cake at 500 kg/ha, seed treatment
with Trichoderma harzianum/T. viride at
4.0 g/kg seed, bacterization of groundnut
seeds with strains of fluorescent pseudomonads or seed treatment with carbendazim
(1.0 g/kg), mancozeb (2.0 g/kg seed) or chlorothalonil/captan (2.0 g/kg) is suggested.
Stem rot caused by
Sclerotium rolfsii Sacc.
The stem rot pathogen has a very wide host
range. In India, stem rot, also known as
Sclerotium wilt, occurs in all groundnutgrowing states and is particularly severe in
Maharashtra and Gujarat. In India, 27% or
more yield loss has been reported. S. rolfsii
also causes indirect losses such as reduction
Sunflower
Sunflower (Helianthus annuus var. macrocarpus (DC) Cockerell) is an important edible oilseed crop. It belongs to the family
Asteraceae. The sunflower head is composed of about 1000–2000 individual flowers. The fertile disc florets bear the seed,
which is white, black or striped grey and
black. The seeds contain 40–50% oil and
50–55% meal, which contain high protein
(35%), calcium, phosphorus, iron, potassium and vitamin E. The sunflower is a
native of North America, where it is used in
dyes, food preparations and medicines.
Alternaria leaf spots and
blight of sunflower
Different species of Alternaria, namely A.
alternata, A. helianthi, A. zinniae, A. helianthiacola, A. leuconthemi and A. tenuissima
266
S.S. Adiver and Kumari
have been reported to cause the disease.
Among these, A. helianthi (Hansf.) Tubaki
and Nishihara are economically important.
The disease has been reported to cause
a huge grain yield loss in Australia, where
yield potential of 1.25 t/ha of the crop was
reduced to 0.1 t/ha (Allen et al., 1981). In
Karnataka, India, the disease occurred in
epidemic form in 1987, with a disease incidence of 95–100% (Hiremath et al., 1990).
The disease is caused by A. helianthi
(Hansf.) Tubaki and Nishihara. The mycelium of the fungus is septate, rarely
branched, brown and 2.5–5.0 µm in breadth.
Conidiospores are cylindrical and yellow to
black grey, with one to 11 transverse septa
and a few longitudinal septa. The conidia
measure in the range of 40–110 × 8–28 µm,
with an average of 74 × 19 µm.
Disease management
Summer deep ploughing reduces the inoculum level in the soil, which is present in
plant debris as dormant mycelium; altering
the date of sowing in order to reduce disease pressure and sowing during August–
September onwards is suggested. Following
spacing 60 × 30 cm under irrigated and
45 × 20 cm under rainfed conditions is recommended. Use of resistant varieties like
GP-145, AH-303, BSH-1 is suggested. Foliar
sprays of mancozeb (0.2%), chlorothalonil
(0.2%), difenaconazole (0.1%) or tebuconazole (0.1%) can prevent the crop from
Alternaria blight.
Rust of sunflower
The pathogen Puccinia helianthi Schw. is a
macrocyclic, autoecious fungus and it produces all the five stages on sunflower only.
The disease has been reported to cause variable yield losses in the crop, depending on
variety, environmental conditions and time
of the outbreak of the disease in the crop
season. Early infection of the variety ‘Sunrise’ and S-37-338 showed 17% and 68%
less yield, respectively. Under field conditions, the disease usually starts at flowering
stage, when the crop attains a dense canopy.
The disease appears in the form of small
cinnamon brown-coloured uredia on the
lower surface of the lower leaves. In severe
conditions, younger leaves, stems, petioles
and floral parts are also infected. When the
crop reaches physiological maturity, most
of the uredia are converted to telia and are
dark brown in colour.
Disease management
Altering the date of sowing reduces disease
pressure. Removal of self-sown plants, crop
rotation for at least 3 years and deep summer ploughing are recommended to reduce
the inoculum level in the soil. Use of resistant varieties like SH-41, SH-187, PH-1, 2, 3,
4, 7 and 8, ICI-306, 331, PAC-36, 9128 and
systemic fungicides containing triazoles,
namely hexaconazole and cyproconazole
(0.1%), are found suitable under field
conditions.
Downy mildew of sunflower
Downy mildew causes heavy yield losses in
sunflower-growing countries of the world.
A serious outbreak (80–90%) of the disease
was recorded in the Red River Valley of
North Dakota and Minnesota (USA) during
1970, resulting in a reduction of about 50%
yield, with a loss of about US$0.5m. Later,
it spread to many European countries, then
to Asia. This spread was mainly through the
seed trade. In India, the disease first appeared
during 1984, in experimental plots of the
Regional Research Station, Latur, particularly during September–October. Later, it
spread to many areas of Maharashtra (Mayee,
1989), Karnataka and Madhya Pradesh
(Agarwal et al., 1991). Causal organisms are
Plasmopara halstedii, P. perennis and P.
patens. The sporangiophores, measuring
150–750 µm, are monopodially branched
almost at right angles and bear zoosporangia
singly at the tips of the branches. Zoosporangia produced from leaves are elliptical
with an apical papilla and measure
17–30 × 15–21 µm. The zoosporangia from
Fungal Diseases of Oilseed Crops
267
roots are uniform, pyriform to oval with 1–3
papillae and 36–66 × 39–40 µm. The sporangia germinate at 5–28°C, with the optimum
temperature being 16–18°C. Zoosporangia
formed at 27°C show 86–95% germination.
Oospores are formed in the intercellular
spaces of roots, stem and seeds and measure
27–32 µm. The fungus causes damping off,
systemic infection and local lesions on
leaves and basal root or stem galls, depending on the stage of infection during plant
growth. Damping-off occurs either as pre- or
post-emergence under damp and cool
weather at seedling stage and gives poor
plant stand (Goosen and Sackston, 1964).
Systemically infected plants remain stunted
with chlorotic leaves.
soft and pulpy, with superficial whitish to
blackish mycelium on the head. Under severe
conditions, rotting spreads to the flower stalk
and the head drops off. Sometimes, the seeds
from the rotted head shed and those that
remain on the head have a bitter taste.
Disease management
Regulatory measures: the pathogen is seedborne and exhibits races; therefore, a quarantine measure has been imposed to check
the movement of virulent races from endemic
areas to other countries. Use of the resistant
variety LDMRSH-1 and seed treatment with
metalaxyl MZ 72 WP at 5–6 g/kg or apron 35
SD at 5–6 g/kg seed is suggested.
Safflower (Carthamus tinctorius L.) is one
of the rabi season oilseed crops cultivated
in medium to heavy textured soils, mainly in
Maharashtra, Karnataka and Andhra Pradesh,
India. Being a crop mostly of the poor smallholder, it came to be recognized as an edible
oilseed crop because of its superior role over
animal fats and other vegetable oils, resulting in a boom in the cultivated area under
the crop.
Rhizopus head rot of sunflower
Alternaria leaf blight of safflower
The disease is caused by three different Rhizopus spp., namely R. nigricans, R. arrhizus and
R. oryzae. The fungal colony is cottony-white
to brown in R. arrhizus, while it is cottonywhite turning brownish-grey to blackish-grey
in R. oryzae and R. nigricans. The optimum
temperature for the growth of R. arrhizus, R.
oryzae and R. nigricans is reported to be 37°C
(thermophyllic), 30°C and 22°C, respectively.
The disease causes severe yield losses, particularly in wet weather conditions. The disease has no effect on seed size but it reduces
seed weight. Affected seeds become scurfy
with discoloration of the hull and partial to
complete discoloration of the nut meal and
the quality of the oil is affected because of
off-flavours. The disease first appears as
brown, water-soaked irregular spots on the
back of the ripening head, usually adjacent to
the flower stalk. The spots enlarge and turn
Leaf blight caused by A. carthami Choudhary is the most destructive disease of safflower in India, appearing in a severe form
wherever the crop is grown and causing up
to 90% reduction in crop yield and oil content of affected seeds. However, the pathogen is reported to increase significantly the
level of free fatty acids in the seeds (Heaton
et al., 1978). Mycelium of the pathogen A.
carthami is septate, inter and intracellular
and dark coloured on maturity. Conidiophores are septate, unbranched, erect and
brown to olivaceous brown, pale near the
apex, measuring 15–85 µm × 6–10 µm, arising through the epidermis or stomata singly
or in clusters. Conidia are light brown to
translucent in shade, with/without a long
beak, showing constrictions at the septa and
borne singly or in short chains. The disease
appears in seedlings on hypocotyls and on
Disease management
Management of insects by spraying endosulphan or diazinon at the onset of bloom
and spraying of fungicide, i.e. carbendazim
(0.1%), on completion of the flowering stage
is effective in controlling the disease.
Safflower
268
S.S. Adiver and Kumari
cotyledons as dark necrotic lesions up to
5 mm in diameter, which may sometimes
result in damping-off. Spots, having concentric rings up to 2 cm in diameter, light to dark
brown with the centre lighter in colour, are
observed in mature plants on leaves and frequently coalesce into large irregular lesions.
Disease management
The disease can be managed by using seeds
from early sown dry land crops and treating
them with thiram and TPTH, captan 0.3%
(Siddaramaiah et al., 1980). Hot water treatment at 50°C for 30 min is also found useful
(Sastry, 1996). Bulb extract (1.0% w/v) of
Allium sativum also shows promise in checking the disease. Varieties like EC-32012, JLA1753, C-2603, Co-1, C75-7218, HUS-524,
476, 305, 260, SSF-112, CTC-251, 248, 252,
etc., are reported to exhibit a variable degree
of tolerance to A. carthami infection.
Fusarium wilt of safflower
Wilt of safflower is caused by Fusarium
oxysporum f.sp. carthami. Fusarial mycotoxins capable of causing mycotoxicoses
have been reported as being produced in
sufficient quantities on infested seeds of safflower in storage (Ghosal et al., 1977). The
disease manifests at all growth stages. It may
cause pre-emergence death or delayed germination of seeds. Symptoms on seedlings
during post-emergence are blackening at the
collar region; chlorotic, small brown spots
appear on cotyledonary leaves, which then
shrivel, become brittle, sometimes get rolled
and droop downwards; finally, the seedlings
bend and die. Plants grown from infected
seeds rarely survive beyond the seedling
stage. In mature plants, lateral branches on
one side may be killed, while the other half
of the plant shows no disease symptoms.
Such plants show partial recovery, but symptoms may reappear later. Sporodochial production on stems may also be visible. Flower
head size is reduced in severely affected
plants, less seeds are formed and many of
them are small, distorted, black and chaffy
(Chakrabarti, 1980; Sastry and Jayaraman,
1993). The mycelium of the pathogen F.
oxysporum f.sp. carthami Klisiewicz and
Houston is septate and branched conidia,
straight or curved, often pointed at the tip
with a rounded base and measure up to
10–36 µm × 3–6 µm. Microconidia are oval
to elliptical, one-celled and measure up to
5–16 µm × 2.2–3.5 µm in size. Chlamydospores are single celled, smooth, faintly
coloured, single or in chains and 5–13 µm ×
10 µm in size. Four biotypes of the pathogen have been identified on the basis of the
reaction of safflower differentials to its isolates (Sastry and Chattopadhyay, 1999).
Disease management
Seed treatment with carbendazim (1.0 g/kg),
captan (2.0 g/kg), thiram (2.0 g/kg) or
Trichoderma (4.0 g/kg) helps to avoid infection of the plant by the pathogen. Crop rotation with legumes like chickpea, cowpea
and pigeon pea helps to manage the disease
(Sastry and Jayaraman, 1993). Use of tolerant varieties HUS-3234, 3123, 305, BSF-3,
CTV-53, etc., is recommended.
Phytophthora root rot of safflower
The mycelium of the pathogen Phytophthora
drechsleri is hyaline, aseptate, branched
and 4.5 µm wide. Sporangia are hyaline to
faint in colour, thin-walled, non-papillate,
pyriform to ovate, 34–38 µm × 15–24 µm in
size and having zoospores measuring
10–20 µm in diameter. Oospores are spherical, smooth, thick-walled, yellow to bright
brown and are 16–45 µm in diameter (Klisiewicz, 1977).
Root rot of Safflower caused by P.
drechsleri Tuck is reported to cause about 3%
losses on average, although 80% losses have
been observed in a few instances, particularly when grown under surface irrigation
(Sastry, 1996). Safflower is affected by
Phytophthora root rot at any stage from
pre-emergence to maturity. Symptoms on
seedlings of 2–3 weeks of age appear as
water-soaked lesions with softening and
collapse of cortical tissue of the lower stem,
whereon the plants lodge, shrivel and die.
Fungal Diseases of Oilseed Crops
Disease management
Draining out excess water from beds after
irrigation and avoidance of monocropping
may help to control the disease (Kolte, 1965).
Use of resistant varieties US-10, Gila, Frio
and VFR-1 is recommended.
269
in light soils during kharif and in heavy
soils during the early rabi season. It occupies an area of 17.50 hundred thousand ha
in India, with production of 587.1 thousand
t. The overall productivity of this crop in
India is 335 kg/ha. About 72 fungi have
been reported on this plant in India (Vyas
et al., 1984).
Rust of safflower
Uredosori of the obligate, autoecious, heterothallic, macrocyclic pathogenic fungus
P. carthami (Hutz) Corda contain numerous
globoid or broadly ellipsoid echinulate, light
chestnut brown uredospores measuring
21–27 µm × 21–24 µm, thick-walled and 3–4
equatorial germpores (Singh, 1998). Safflower rust caused by P. carthami, is an
important disease in India. It causes a stand
loss of 55–97% in susceptible varieties with
considerable yield loss, particularly if the
infection starts early in the crop growth. The
first pathological phase of the safflower rust
is seen in the seedling stage of the crop, when
orange to yellow spots representing pycnia
appear on cotyledons, which ultimately
leads to drooping and wilting of the plants.
With the development of uredospores and
teliospores, the colour of the spots later
changes to brownish black. The second
pathological phase of the rust is uredia development on leaves, flowers and fruits, where
teliospores are formed later towards crop
maturity when the atmospheric temperature
rises (Schuster and Christiansen, 1952).
Disease management
Destruction of the infected host, crop debris
and collateral host Carthamus oxycantha
(Pohli weed) and crop rotation checks the
disease to some extent. Three sprays of tridemorph (0.5%), thiophanate methyl (0.15%)
or tridimefon (0.1%) are effective against safflower rust (Singh et al., 1997). Use of resistant
varieties APPR-1 and APPR-3 is suggested.
Sesamum
Sesame (Sesamum indicum L.) is an important oilseed crop of India. It is grown mainly
Phytophthora blight of sesame
Phytophthora blight is caused by P. parasitica var. sesame. It was first reported from
India by Butler (1918). Now, it has become
an important disease of sesame and has
been reported from the Dominican Republic
(Ciferri, 1930) and Argentina (Frezzi, 1950).
In India, it was severe in Madhya Pradesh
Rajasthan, Uttar Pradesh and Gujarat
(Vasudeva, 1961; Verma, 2002). This disease
has caused 66% losses in Gujarat (Kale and
Prasad, 1957) and 79.8% in Central Madhya
Pradesh (Singh et al., 1976). It may cause
even 100% loss under the most favourable
conditions for infection to occur severely at
seedling stage. Disease occurs on all the
aerial plant parts. The symptoms of the disease appear as brown, water-soaked spots
on the leaves of seedlings at a very early
stage. Gradually, the spots increase in size.
Under favourable weather conditions, the
whole leaf rots and becomes black. Rotting
progresses further and the whole stem is
rotted. Frequently, the attack on the seedling starts at the collar region and gives
damping-off like symptoms. The cottonywhite growth of the fungal mycelia appears
on the lower side of the leaves and on pods
under humid condition.
Disease management
Intercropping with soybean, castor, maize,
sorghum and pearl millet in the ratio of 1:3
or 3:1 shows a low incidence of the disease,
with a higher yield. Application of FYM
alone or neem cake with inorganic fertilizer
(N60, P40, K20) reduces the disease as compared to without FYM. Application of the
species of Pseudomonas, Bacillus and Streptomyces, which are most active at 25–27°C
270
S.S. Adiver and Kumari
at field capacity moisture level, can be suppressive to Phytophthora species in soil. Seed
treatment with vitavax (1.0 g/kg) and captan
(2.0 g/kg) controls seedling disease effectively.
Captan 75D is the best fungicide for reducing
the disease, followed by thiram 75D.
Fusarium wilt of sesame
Fusarium wilt of sesame is quite serious
wherever the crop is grown. In India, it has
been reported from all the sesame-growing
areas, such as Madhya Pradesh, Maharashtra, Andra Pradesh, Rajasthan, Haryana, Punjab, etc. The disease is quite serious when it
starts in the early stages of crop growth. The
causal organism is F. oxysporum f.sp. sesami. The fungus produces profuse light pink
mycelial growth on PDA. Microconidia are
hyaline, ovoid to ellipsoid, unicellular and
produce abundantly even on the medium
and are about 8.5 × 3.25 µm in size. The
macroconidia are produced abundantly in
sporodochia and size ranges from 35 to
49 × 4.5 µm. The chlamydospores are globose to subglobose, smooth or wrinkled and
about 7–16 µm in diameter. The pathogen
grows at a temperature range of 10–25°C,
with an optimum temperature of 26°C and a
pH of 5.6. The initial symptoms of the disease appear as yellowing of the leaves, which
later droop and desiccate. On the infected
plant, the leaves may show inward rolling of
the edge and eventually may dry up. If the
disease appears at the later stages of crop
growth, the symptoms may appear on one
side of the plant, resulting in partial wilting.
Discoloration of the vascular system is conspicuous in the roots.
Disease management
Seed treatment with benlate (1.0 g/kg) and
vitavax (1.0 g/kg) is most effective against
wilt. Application of conidial dust of Gliocladium virens gave better disease control. Similarly, application of T. harzianum and T.
viride in the field also reduced the incidence
of wilt significantly. Soil drenching with
antibiotic KB-8A isolated from B. polymyxa
at a concentration of 13 µm/ml inhibited
F. oxysporum f.sp. sesami completely (Hyun
et al., 1999).
Alternaria leaf spot of sesame
The pathogen is A. sesame (Kawamura)
Mohanty and Behera. The conidiophores of
the pathogen are pale brown, cylindrical,
erect, not rigid and arise singly with a size
of 30–54 × 4–7 µm. Conidiophores produce
conidia at the apex, which are in chains of
one to two. The conidia are straight or
slightly curved, obclavate, yellowish brown
to dark brown in colour and measure
30–120 × 9–30 µm. The disease affects all the
aboveground plant parts. The initial symptoms appear as small, brown, round to irregular spots on the leaf blade. Later, the spots
enlarge and turn dark with concentric rings.
On the lower surface of the leaves, spots are
light brown in colour. The appearance of the
disease at the seedling stage can cause postemergence damping-off. On capsules, small,
brown spots appear which result in the formation of shrivelled and deformed seeds.
Disease management
Application of Bordeaux mixture (0.1%)
and zineb (0.1%) has been reported to be
effective. Application of mancozeb (0.2%)
at the time of disease initiation is effective
in managing the disease.
Powdery mildew of sesame
This disease is common, especially in South
India. It has been reported that powdery
mildew of sesame is caused by Oidium erysiphoides, Leveillula taurica (Lav.) Trnaud,
Sphaerotheca fuliginea (Schlecht) Pollacci
and Erysiphe cichoracearum DC. The disease causes considerable losses in yield,
depending on the time of its appearance, as
well as the intensity of the disease. Powdery
mildew causes a loss of 42%; every 1%
increase in disease intensity results in a yield
loss of 5.63 kg/ha. Four different fungi have
been reported to cause powdery mildew,
Fungal Diseases of Oilseed Crops
but in India E. cichoracearum is predominantly prevalent. Both conidia and ascospores
on germination give rise to an abundant
superficial mycelium of uninucleate cells,
which form a white coating on the leaf and
send haustoria into the host. The disease normally appears after 45–60 days. The initial
symptoms appear as dirty whitish fungal
patches on the upper surface of the leaves.
Later, these specks coalesce to cover the entire
leaf and result in premature defoliation. Generally, it affects the leaves but in severe cases,
the disease spreads to petioles and other plant
parts. In severe infection, pods or capsules are
shrivelled and produce smaller seeds.
Disease management
Two sprays of wettable sulphur (0.3%),
dinocap (0.1%) or hexaconazole (0.1%)
at 15-day intervals can help to control the
disease.
Cercospora leaf spot/white
leaf spot of sesame
Mycelium of Cercospora sesami Zimmerman is yellowish-white in colour and produces profuse conidiophores in culture.
The conidiophores are olivaceous, septate,
usually single but sometimes up to 10, epiphyllous, nodulase, thickened towards the
tip, conidia with 7–10 septa and measure
about 90–135 × 3–4 µm. Generally, the symptom of the disease appears at the time of flowering, but the disease may also appear after
30–40 days after sowing. The initial symptoms of the disease are circular spots scattered
on both leaf surfaces. These spots enlarge rapidly and become up to 5 mm in diameter. The
spots are initially brown in colour with a
whitish centre, but later they may be brown to
dark brown in colour. The symptoms on petioles are visible as elongated lesions, whereas
on capsules they are more or less circular and
brown to dark brown in colour.
Disease management
Three sprays either of carbendazim (0.05%)
and topsin M-70 (0.2%) at 10-day intervals
271
were best in controlling the disease. Resistant varieties recommended are BIC-7-2,
Sidhi-54, Rewa-114 and Seoni Malwa.
Castor
Castor (Ricinus communis L.), belonging to
the family Euphorbiaceae, is the most
important non-edible oilseed crop of arid
and semi-arid regions of India. Castor oil finds
its application in the manufacture of a wide
range of ever expanding industrial products,
such as nylon fibres, jet engine lubricants,
hydraulic fluids, dyes, detergents, soaps, ointment, greases, paints, varnishes, cosmetics
and perfumes, etc. (Pathak, 2003).
Castor is grown in tropical and subtropical climates; the major growing countries
are India, China and Brazil. India occupies
about 57% of the world castor acreage, but
produces about 62% of world production.
The major castor-growing states in India are
Gujarat, Andhra Pradesh, Tamil Nadu and
Orissa. Productivity is highest in Gujarat
state because more that 90% of the cultivated
area is covered by castor hybrids under irrigation. There are a number of diseases occurring on castor and the important ones are
explained below.
Alternaria blight of castor
This is caused by A. carthami (Yoshii) Hansford. The disease appears on leaves, stem,
inflorescence and capsules. At seedling
stage, light brown spots first appear on cotyledonary leaves, which become angular with
age. Severe infection results in the death of
young seedlings or foliar blight. Symptoms on
adult plant leaves are brown, zonate and variable in size and usually surrounded by yellow
halos. In the case of severe infection, premature defoliation occurs. Sunken spots develop
on capsules on one side, which gradually
enlarge to cover the whole capsule with fungal growth. Such capsules are smaller in size
and have underdeveloped or wrinkled seeds
with little oil content. In heavily infected
field crop, all the young racemes and even
flower primordia are killed.
272
S.S. Adiver and Kumari
Disease management
Foliar application of mancozeb (0.2%) at
intervals of 15 days starting from the appearance of the disease is beneficial. Judicious
use of nitrogenous fertilizers also reduces
the development of the disease.
Botrytis grey rot of castor
This is a very serious disease of castor as it
affects the flowers and capsules directly and
the entire crop may be lost if there are continuous rains during capsule formation. The
disease is confined to only a few states in
India and is serious in Andhra Pradesh and
Tamil Nadu. It is caused by Botrytis ricini
Godfrey. The disease is confined to spikes
or racemes. Generally, pale to olive grey
coloured woolly growth of the fungus is
observed on flowers or capsules. The disease
appears initially as small blackish spots,
exuding a drop of yellow liquid. Fungal
infection from these spots further spreads to
racemes. The infected flowers appear soft
due to the profuse growth and sporulation
of the pathogen. This later turns to grey
masses covered with dusty powder, resulting in the rotting of capsules. The unripe
seed becomes soft and mature ones hollow,
resulting in a discoloured seed coat and loss
in seed weight.
collar rot, root rot and twig blight. The disease appears at different phases as collar
rot, stem blight and root rot. Initially, the
infected plant shows signs of water shortage. Within a week, the leaves and petiole
droop and finally, within a fortnight, the
entire plant dries up and can be pulled up
easily. Collar rot phase is observed 30–40
days after sowing. Dark black discolorations
are seen at the collar region of the plant,
which gets sunken and later becomes abnormal. The affected tissue becomes shredded
and weak and finally shows sign of wilting.
Stem blight symptoms appear slightly later,
due to aerial infection, as straw-coloured or
brown depressed small lesions on the stem,
usually at the nodes. The lesions increase in
size by both upward and downward extension of the infection, resulting in a 2–20 cm
oval-shaped necrotic area. The surface of
the infected stem shrinks at this region and
the plant breaks easily at this point. The
affected spikes are discoloured, turn black
and dry up in the course of time. Infected
capsules become discoloured and drop off
easily. In the case of the root-rot phase, the
taproot shows signs of drying and the root
bark shreds off easily. Rotting sometimes
spreads partly above the ground. At an
advanced stage, sclerotial bodies may be
seen as minute black dots on the surface of
woody tissues and in the pith region.
Disease management
Disease management
Adoption of wider spacing with varieties
having open racemes reduces the severity of
the disease. Two prophylactic sprays of carbendazim (0.05%), one at 50% flowering
and the other soon after the appearance of
the disease, reduces incidence of the disease effectively.
Crop rotation with non-host crops and
mixed cropping with moth bean can be
helpful in reducing the disease. Infected
plant material should be collected and
burnt. Application of thiram (2.0 g/kg) or
carbendazim (1.0 g/kg) as seed dresser along
with spray and soil drench is recommended.
Topsin M-70 has also been found effective
for controlling root-rot disease in castor.
Macrophomina root rot of castor
Wilt of castor
Macrophomina phaseolina (Tassi) Goid is
reported to cause different symptoms on
castor, namely seedling blight, dieback due
to aerial infection, spike blight, stem blight,
Wilt of castor is caused by F. oxysporum
f. sp. ricini Nanda and Prasad. The extent of
disease incidence has been up to 80% in
Fungal Diseases of Oilseed Crops
Russia (Moshkin, 1986). Losses in yield were
realized in all cultivated castor hybrids in
Gujarat and up to 85% incidence of the disease has been reported in North Gujarat
(Dange et al., 1997). Young seedlings at the
two- to three-leaf stage exhibit discoloration
of hypocotyls and loss of turgidity, with or
without change in colour. The mycelium
penetrates the vascular system of the roots,
stems and leaves causing necrosis, which
leads to wilting and finally death of the
plant. At the time of flowering and spike
formation stages, the disease is characterized by a gradual yellowing and shrivelling,
with marginal and interveinal necrosis of
leaves. Infected plants rarely bear seeds and
such seeds are deformed and light in weight.
Roots of wilted plants show blackening and
necrosis, while in the case of partial wilted
273
plants, only one side of the root system is
observed as being blackish and necrotic; the
other side of the root system remains healthy.
When the stem of the wilted plant is split
open, a white cottony fungal growth is
observed in the pith region, which then
becomes blackish.
Disease management
Use of healthy seeds, crop rotation, summer
deep ploughing and field sanitation reduce
the incidence of the disease. Use of bioagents like T. harzianum and T. viride have
been screened for their antagonistic activity
against castor wilt pathogen. Seed treatment
(1.0 g/kg) and pre-sowing soil application of
carbendazim at 3.0 kg a.i./ha with thiram
(3.0 g/kg seed) is recommended.
References
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Diseases. CRC Press, Inc, Florida, 135 pp.
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21
Occurrence of Pyrenophora
tritici-repentis Causing
Tan Spot in Argentina
M.V. Moreno1,2 and A.E. Perelló2,3
1Laboratorio
de Biología Funcional y Biotecnología, CEBB, Facultad de Agronomía
de Azul, Universidad Nacional del Centro de la Provincia de Buenos Aires,
Buenos Aires, Argentina; 2Consejo Nacional de Investigaciones Científicas
y Técnicas (CONICET); 3Centro de Investigaciones de Fitopatología (CIDEFI),
Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata,
La Plata, Provincia de Buenos Aires, Argentina
Abstract
Wheat (Triticum aestivum L.) is currently considered as one of the most important crops worldwide.
It can be affected by several diseases. However, only a limited number of them, like ‘tan spot’ resulting
from the fungus produced by Pyrenophora tritici–repentis, cause serious problems to the crop and may
be given special attention. Tan spot has significant economic consequences. In recent years, the
incidence of the disease has increased in many areas where wheat is cultivated, becoming a serious
problem by causing losses of up to 70%. It has been found in a lot of countries worldwide: North
Dakota, Nebraska and Kansas (USA), Canada, Australia, Asia, Pakistan, Czech Republic, Poland,
Ukraine, Hungary, France, Denmark and Belgium.
This disease has increased its incidence, prevalence and severity, particularly in the whole of the
South Cone region in the last few years: Argentina, Brazil, Bolivia, Colombia, Ecuador, Peru, Paraguay
and Uruguay. Tan spot is one of the most destructive and widespread problems of wheat production
in Argentina. In this chapter, we summarize the knowledge of many and diverse contributions and we
highlight what is known and unknown about the disease.
Introduction
Wheat is considered one of the most important crops of the world, along with rice,
maize and potato. Humans consume around
75% of worldwide production (Wiese, 1987;
Rajaram, 2001). In the period between 1970
and 2000, wheat yields rose at an annual
rate of 2.3%; however, the area cultivated
remained the same. The volume of wheat
traded is greater than any other grain (Eikboir
and Morris, 2001).
Wheat has been one of the most important crops for the past 100 years in Argentina. Between 2005 and 2006, 2m ha were
cultivated, giving a yield of 70–74 Mt (Inf.
Económico de Coyuntura No. 261, 2006).
The future of this production and South
America’s participation in the international
market depended on how Argentina and
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
275
276
M.V. Moreno and A.E. Perelló
Brazil developed the crop. In Argentina,
planting in the field to exportation of the crop
was dependent on the introduction of new
technological developments. These developments include new cultivar composition,
management of the crop and the management of future areas to expand yield (Eikboir and Morris, 2001). Fungal pathogens
are the result of a combination of these factors (Klein, 2001). Management of these diseases requires specific knowledge and an
increased ability to identify the fungus and
techniques to reduce crop losses to a minimum (Kohli, 1995).
In the last few years, minimum tillage
has been considered advantageous to soil
conservation, but it leads to a loss of available nutrients and a potential increase in
necrotic pathogens whose saprophytic stage
lives in the straw of the crop (Annone,
1985). Establishment of the crop under this
management can be affected by pathogens of
this type (Table 21.1). In Argentina, the
increased incidence in leaf spot since the
application of minimum tillage has been a
cause for concern (Annone and Kohli, 1996).
Tan Spot
The first time that tan spot was observed on
wheat was in the 1920s in Japan (Hosford,
1981). In 1954, the first loss (75%) was
reported in Kenya (Gilchrist et al., 1984). In
1940, tan spot was reported for the first time
in the USA (Barrus, 1942). At the present
time, the name of the fungus causing tan
spot is reported with high frequency in the
wheat-growing areas of the world (Conners,
1939; Tekauz, 1976; Watkins, et al., 1978;
Sim and Willis, 1982; Loughman et al.,
1998, Postnifova and Khasanov, 1998; Ali
and Francl, 2001a; Sarova et al., 2002).
In South America, tan spot has been
observed in Colombia, Ecuador and Peru
(Dubin, 1983). It has recently gained predominance among wheat diseases in the
Southern Cone region of South America,
comprising Argentina, Brazil, Chile, Paraguay and Uruguay (Kohli et al., 1992; Linhares and da Luz, 1994).
The disease began to affect wheat crops
noticeably in the north-central region of the
Buenos Aires Province in the early 1980s
(Annone, 1985, 1996). Since then, tan spot
symptoms have been detected in most
growing areas of the country. The disease
is particularly prevalent and intense in
the northern area of the Argentine wheatproducing region (central and northern
Buenos Aires, southern Santa Fe, southeastern Cordoba and Entre Rios Provinces),
where highly conducive environmental
conditions and increasing use of minimum
tillage have created a disease hotspot. In the
region, the pseudothecia of the pathogen are
formed on wheat residue left on the soil surface at crop sowing and/or early growth
stages. Conidia are formed and released
soon after the development of the first symptoms on leaves (Annone et al., 1994).
Wright and Sutton (1990) observed that
when P. tritici-repentis was introduced in
an area of wheat, it was dominant over other
leaf pathogens. In Argentina, tan spot is one
of the most important diseases, along with
rust and head blight (Annone, 2006). The
massive expansion of minimum tillage in
Argentina has encouraged the establishment and development of this disease
(Annone and García, 2004).
Importance
Tan spot is frequently observed in most
farmers’ fields, often affecting the upper
leaves at flowering to early grain filling
stages. Yield losses of between 9 and 50%
have been observed by several authors (Hosford and Busch, 1974; Sharp et al., 1976;
Rees et al., 1982; Rees and Platz, 1983). In
South America, yield losses of around 40%
were observed by Mehta and Gaudencio
(1991) in Brazil and Kohli et al. (1992)
reported wheat yield losses of between 20
and 70% in Paraguay and Argentina. Estimates of losses (10–20%) caused by the disease have been made by comparing fungicides
protected with non-protected wheat plots
(unpublished data Annone, 1996). Similar
results were obtained by Galich and Galich
Tan Spot in Argentina
277
Table 21.1. Diseases of wheat and the agent cause (Annone and Kohli,
1996).
Pathogen
Disease
Xanthomonas campestris pv. undulosa
Septoria tritici
Drechslera tritici-repentis
Fusarium graminearum
Gaeumannomyces graminis var. tritici
Bacterial stripe
Spot blotch
Tan spot
Head blight
Take-all
(1994) in Marcos Juarez (Cordoba Province).
They determined that losses due to tan spot
associated with Septoria tritici blotch ranged
between 6 and 13.5%. Tan spot is a complex
disease that is dependent on its geographic
location and the environmental conditions
prevailing (de Wolf and Francl, 1998).
The pathogen
The tan spot fungus is an Ascomycota currently known as P. tritici-repentis (Ptr)
(Died.) Drechs. It is a facultative pathogen
whose asexual stage is Drechslera triticirepentis (Dtr) (Died.).
P. tritici-repentis was isolated for the
first time from Agropyron repens in Germany and it was named Pleospora trichostoma by Diecke. In 1928, it was isolated
from wheat by Nisikado (Nisikado, 1928),
when it was named Helminthosporium
tritici-repentis (= Drechslera tritici-repentis)
(Hosford, 1981).
The genera Pyrenophora Fr. was used
frequently for some ascomycota parasitic on
cereals and other grasses (Diaz de Ackerman,
1987). It was described by Fries in 1849 and
cited by Shoemaker in 1961 (Shoemaker,
1962). In 1869, Fuckel noted the tendency
of P. phaecomes to mature only after overwintering and found a Drechslera conidial
stage of P. phaecomes (Shoemaker, 1962).
In 1883, Saccardo used the presence of setae
on the ascocarp of Pyrenophora and the
absence of setae on the ascocarp of Pleospora to separate these two genera. In 1885,
Winter (Shoemaker, 1962) included the
species of both genera in Pleospora and in
1934, Drechsler (Shoemaker, 1962) agreed
with Winter that the presence or absence of
setae was not an important enough characteristic to separate the two genera. He
emphasized the connection between Pyrenophora and the conidial stage Drechslera, as
found by Fuckel for P. phaecomes. Drechsler
too determined the connection between P.
teres and D. teres, P. tritici-repentis and D.
tritici-repentis, and P. bromi and D. bromi
(Shoemaker, 1962). At the same time, Ito and
Kuribayaski (1931) connected five species
of Pyrenophora with the conidial stage of
Drechslera. In 1949, Wehmeyer worked on
the distinction in form and size of the Pleospora and Pyrenophora ascospores (Wehmeyer, 1949).
In 1930, Ito described the genera
Drechslera. In 1809, Link described the genera Helminthosporium, where species of
Drechslera were included (Ito, 1930). In
1902, Diedicke (Drechsler, 1923) determined
H. tritici-repentis as formae of H. gramineum.
In 1923, Drechsler recognized H. teres, H.
bromi, H. gramineum and D. avenae as unique
species. In 1959, Shoemaker (1962) made the
distinction between two subgenera, CylindroHelminthosporium, in which all the species
have conidia germinating from all cells and
Eu-Helminthosporium, in which all the species have fusiform conidia germinating from
end cells only. In 1930, Ito (Shoemaker, 1962)
proposed the name Drechslera for those species with cylindric conidia germinating from
all cells, using as a type D. tritici-repentis. He
used the name Bipolaris for those species
whose conidia were fusiform, germinating
from end cells only. In 1962, Shoemaker
considered D. tritici-vulgaris as D. triticirepentis. Currently, the teleomorphic nomenclature of the fungus is P. tritici-repentis
278
M.V. Moreno and A.E. Perelló
and the anamorph of the fungus is unanimously accepted as D. tritici-repentis. Morphological data can be found in Drechler
(1923), Shoemaker (1962) and Wehmeyer
(1954).
Host–Parasite Interactions
Symptomatology. On susceptible wheat leaves,
P. tritici-repentis(Ptr) produces characteristic
oval to diamond-shaped lesions. However,
newly formed tan spot lesions cannot be separated reliably from those caused by other
necrotrophic pathogens. Later, lesions elongate and develop a tan colour with a chlorotic halo and a small dark brown infection
site. Chlorotic areas tend to coalesce on heavily infected leaves, especially on young
plants, a symptom which leads to the disease
name, ‘yellow leaf spot’ (Fig. 21.1). On resistant and partially resistant wheat, lesion size
is reduced and chlorosis and necrosis may
be absent (de Wolf et al., 1998).
Lamari and Bernier (1989a) identified
two different types of symptoms produced
by the pathogen: tan necrosis and extensive
chlorosis. However, they reported that the
pathogen isolates could be characterized by
their ability to induce tan necrosis and/or
chlorosis. They grouped the isolates into
four pathotypes based on the production of
different symptoms on different lines. In
this system, an unlimited number of isolates
were designated as races 1, 2, 3, 4, 5, 6, 7, 8,
(a)
Fig. 21.1.
9, 10, 11 and 12 (Lamari and Bernier, 1989a,b;
Lamari et al., 1995, 1998, 2003, 2005;
Lamari and Gilbert, 1998; Ali and Francl,
2001a,b, 2002a,b). Races 9 and 10 have been
identified in South America, which indicates that the Ptr population is heterogeneous in this area (Ali and Francl, 2002b).
In Argentina, the race population structure
is unknown and in 2007, Moreno observed
that isolates obtained from Argentina produced three reaction types on cultivars of
local and international wheat (Moreno,
2007). Actually, the isolates were inoculated on different wheat sets to determine
the races present in Argentina.
Ptr can also infect wheat seed during
the grain-filling period (Schilder and Bergstrom, 1994). This disorder is called red
smudge, because infected seed has a reddish
discoloration (Valder, 1954).
Disease cycle
Dispersal and infection by Ptr can develop
between 10° and 30°C with moisture
between 6 h and 48 h (Larez et al., 1986;
Hosford et al., 1987; Sah, 1994). These conditions are the reason why tan spot can
occur all year round and which distinguishes it from the white head disease, but
they all depend on environmental conditions (Carmona, 2003).
The disease cycle of tan spot (Fig. 21.2)
provides a convenient framework on which
(b)
Pyrenophora tritici-repentis produces characteristic oval to diamond-shaped lesions.
Tan Spot in Argentina
279
Symptoms on leaf tissues
Primary infection
Conidia
Secondary host
Secondary infection
Seeds
Primary infection
Ascas
Fruiting bodies on stem
Fig. 21.2. Disease cycle of Pyrenophora tritici-repentis, agent cause of tan spot of wheat.
to explain our current understanding of the
progress of the disease. The rate of progression through the disease cycle depends on
the host and on temporal and environmental components of the pathosystem (de Wolf
et al., 1998).
The seeds, straw and collateral hosts
are the principal source of inoculum of tan
spot. The primary inoculum can travel long
distances through the wheat-growing areas
and is introduced into new areas by seeds.
In the seed, the pathogen lives in the
pericarp as mycelium and transmission
to the rest of the plant is non-systemic
(Schilder and Bergstron, 1994). In Argentina, Barreto (1984, unpublished data)
observed infection on 2% of wheat seed.
Future investigations are required to establish the sanitatary management of seeds
(Carmona, 2003).
Another source of primary inoculum is
wheat straw. Several authors consider straw
as the principal source of the inoculum of
Ptr (Rees and Platz, 1980).
Collateral hosts of Ptr could play
an important role as a source of primary
inoculum between growing seasons, as a
source of genetic variation and as a reservoir of a fungal population genetically different than that prevalent on wheat (de Wolf
et al., 1998). The tan spot fungus has been
reported on many grass species from different parts of the world, among which are
Agropyron sp., Avena fatua, A. sativa, Echinochloa sp., Elymus innovatus, Andropogon
gerardi, Alopecurus arundinaceus, Bromus
inermis, Dactilys glomerata, Lolium perenne,
Phalaris arundinaceae, Poa sp. and Secale
cereale (Diedicke, 1902; Drechsler, 1923;
Conners, 1939; Dennis and Wakefield, 1946;
Sprague, 1950; Andersen, 1955; Dickson,
1956; Shoemaker, 1962; Hosford, 1971; Howard and Morral, 1975; Farr et al., 1989;
Krupinsky, 1992c; Ali and Francl, 2002b).
In Argentina, the host range is unknown.
Ascospores are generated in the pseudothecia that live in the wheat straw. The
conidia are formed in the straw containing the
pseudothecia and on the leaves of infected
plants or the leaves of collateral hosts.
The ascospores of Ptr are dispersed primarily by wind, but the distance an ascospore
280
M.V. Moreno and A.E. Perelló
can travel is limited (Schilder and Bergstrom,
1995). Limitations on ascospore dispersal
distance have been attributed in part to
short discharge distances from the pseudothecia. However, it is doubtful that the short
discharge distance alone can account for
these short dispersal distances. Schilder
and Bergstrom (1992) proposed that movement was limited during periods of high
relative humidity when ascospores were
discharged from the ascocarps (de Wolf
et al., 1998). Infested residue usually results
in significant disease severity at flag leaf
emergence and later growth stages due to
secondary infections (McFadden and Harding, 1989; Wright and Sutton, 1990; McFadden, 1991).
Following liberation from the host, the
conidia of Ptr can be sampled readily during aerial dispersal and differentiated successfully from other fungi (Morral and
Howard, 1975; Rees and Platz, 1980; Wright
and Sutton, 1990; Krupinsky, 1992b; Maraite
et al., 1992; Schilder and Bergstrom, 1992;
Wolf and Hoffmann, 1993). Morrall and
Howard (1975) reported that conidia numbers reached their highest levels late in the
growing season and that the number of
conidia has a clear diurnal periodicity. The
numbers of conidia of the pathogen decline
sharply with dispersal distance. Schilder
and Bergstrom (1992) reported that the highest number of conidia occurred within 3 m
of the inoculum source and that 60–100%
of the recoverable conidia were sampled
within 25 m. Only a few conidia could be
recovered 100 m away from the inoculum
source, but this suggested that longer dispersal distances were possible. When the
conidia were deposited on the leaf, their germination was influenced by both temperature and the availability of free moisture
(Mihtra, 1934). The conditions that contribute to infection by Ptr in an outdoor environment have also been studied (Ali, 1993;
Francl, 1998; de Wolf and Francl, 1997). The
precise range of temperatures optimal for
disease development varies with cultivar
(Luz and Bergstrom, 1986). Leaf age affects
the severity of the disease caused by Ptr
(Cox and Hosford, 1987; Lamari and Bernier,
1989a,b; Hosford et al., 1990; Perelló et al.,
2003a). The growth stage seems to influence
tan spot severity and expression of resistance (Hosford et al., 1990; Fernandez et al.,
1994; Perelló et al., 2003a).
Then, infecting the wheat leaves, conidia
are produced and the pathogen’s asexual
cycle life develops by infecting new plants of
wheat. Even so, conidiogenesis continues in
wheat straw. The production of conidia and
the development of pseudothecia depend
on temperature and water potential. Stem
colonization appeared to be the result of the
progressive colonization of the leaf sheath
and upper internode. No differences in saprophytic colonization were observed among
cultivars of varying resistance (de Wolf
et al., 1998). Numerous researchers have
investigated the factors affecting the initiation and development of pseudothecia in
laboratory experiments (Odvody et al., 1982;
Pfender and Wootke, 1987; Pfender et al.,
1988; Summerell and Burgess, 1988a,b,
1989; Zhang and Pfender, 1993).
The effects of water potential in wheat
straw on pseudothecial development have
also been studied in an outdoor environment
(Fernandes et al., 1991; Zhang and Pfender,
1993). The number of ascocarps per gram of
straw in near-soil straw was 32% and 42%
of that found in mowed and no-till treatments, respectively. In addition, the number of ascocarps produced in the lower
portion of standing stubble of no-till plots
was 12% of the number found in the upper
portion.
Reports of pseudothecia maturation in
an outdoor environment vary from region to
region (Rees and Platz, 1980; Odvody et al.,
1982; Summerell and Burgess, 1988b, 1989;
Wright and Sutton, 1990; Wolf and Hoffmann, 1993). In most regions where wheat
is grown, the pseudothecia of Ptr are initiated when the crop has reached full maturity and begins to senesce (Odvody et al.,
1982; Wolf and Hoffmann, 1993). However,
in colder climates, pseudothecia may not be
initiated until the following growing season
(Fernandez et al., 1998).
In Argentina, the sexual stage of Ptr has
been detected in wheat straw, but it is
unknown in which regions and under what
conditions development took place.
Tan Spot in Argentina
Physiological Specialization
The terms ‘pathogenicity’ and ‘virulence’
are likely to be used to describe the ability
of an organism to cause disease. Pathogenicity is regarded as a general attribute of a species, while virulence is an attribute reserved
for a particular strain of a pathogen in relation to a particular host genotype (Day, 1960).
There exist virulent races of Ptr that interact
with wheat hosts in a highly specific manner. This suggests that host-specificity attributes are superimposed on the general
pathogenic ability of Ptr.
Variation in virulence in the population of this pathogen is essential in understanding the interaction of the genomes
involved in tan spot. Studies of the diversity of virulence within a pathogen population should help in the development of a
successful disease management programme,
particularly resistant cultivars. Several investigators have described diversity among Ptr
isolated from different areas around the
world (Christensen and Graham, 1934; Misra
and Singh, 1972; Luz and Hosford, 1980; Gilchrist et al., 1984; Krupinsky, 1987, 1992a,b;
Diaz de Ackermann et al., 1988; Lamari and
Bernier, 1989a; Schilder and Bergstrom,
1990; Ali and Buchenau, 1992; Sah and Ferhmann, 1992; Brown and Hunger, 1993;
Moreno, 2007).
In 1971, Hosford observed differences
between the reaction type on wheat cultivars produced by isolates of Ptr. Misra and
Singh (1972) tested isolates originating from
India and they detected significant differences in virulence, based on lesion size.
Some results were observed by Gilchrist
et al. (1984) when they tested isolates collected from Mexico on the wheat cultivar
Morocco. Luz and Hosford (1980) grouped
the isolates tested into 12 races based on
statistical mean separation. However, Díaz
de Ackermann et al. (1988) did not find any
difference in virulence among the isolates
tested by Luz and Hosford (1980). Hunger
and Brown (1987) tested nine isolates originating from the USA; these isolates showed
significant differences on the susceptible
cultivar TAM 105. Krupinsky (1987) showed
281
differences in lesion length and percentage
of severity among isolates of Ptr obtained
from Bromus inermis.
Lamari and Bernier (1989a) grouped
the isolates of Ptr into three pathotypes on 11
cultivars of wheat based on the type of reaction. Schilder and Bergstrom (1990) tested
70 isolates obtained from Canada on 12
wheat cultivars and detected significant differences among the interaction of isolate ×
cultivar. Some results were reported by Sah
and Ferhmann in 1992 for isolates originating from Brazil, Germany, India, Nepal and
the USA. However, Krupinsky (1992a,b)
detected variation among levels of aggressiveness but he found no differences in levels of virulence. In 1992, Ali and Buchneau
observed physiological specialization based
on the reaction type for isolates obtained from
the USA. Mehta et al. (2004) tested 40 isolates
obtained from Parana (Brazil) on six wheat
cultivars; they observed low interaction for
isolate × cultivar. In 2007, Moreno detected
significant differences in isolate × cultivar for
isolates of Ptr obtained from wheat-growing
areas in Argentina.
Races 1, 2, 3 and 4 of Ptr correspond
with those determined by Lamari et al.
(1995). Races 1 and 2 are predominant in
North America (Ali and Francl, 2003). The
greater part of isolates identified as race 5
originate from North Africa, North America
and Azerbaijan (Ali et al., 1990; Lamari
et al., 1995, 1998; Strelkov et al., 2002; Ali
and Francl, 2003). Races 6, 7 and 8 were
identified from collections originating from
Algeria, Caucaso and South America (Ali
and Francl, 2002a; Strelkov et al., 2002;
Lamari et al., 2003). Finally, races 9 and 10
were identified from isolates originating
from South America (Ali and Francl,
2002a,b).
These studies indicate that variation in
the pathogen population can be detected by
using either quantitative or qualitative rating
scales (Table 21.2). Research using quantitative scales generally detected variation in
virulence on susceptible lines, but isolates
in different studies produced an equal reaction on resistant cultivars (de Wolf et al.,
1998).
282
Table 21.2.
M.V. Moreno and A.E. Perelló
Relationships between pathotypes, races and wheat cultivars.
Cultivars/lines of wheat
Races
Glenlea
Katepwa
6B662
6B365
Salomouni
M3
1
2
3
4
5
6
7
8
N (Tox A)
N (Tox A)
R
R
R
R
N (Tox A)
N (Tox A)
N (Tox A)
N (Tox A)
R
R
Cl (Tox B)
Cl (Tox B)
N (Tox A) Cl (Tox B)
N (Tox A) Cl (Tox B)
R
R
R
R
Cl (Tox B)
Cl (Tox B)
Cl (Tox B)
Cl (Tox B)
Cl (Tox C)
R
Cl (Tox C)
R
R
Cl (Tox C)
R
Cl (Tox C)
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Note: R, resistance; N, necrosis; CL, chlorosis; Tox A, presence of Tox A and production of Tox A; Tox B, presence of Tox
B and production of Tox B.
In Argentina, the race population structure is unknown. Future research studying
physiological specialization in Ptr should
consider collections originating in Argentina.
Disease Management Strategies
From the point of view of the disease’s
development, its management is achieved
in different ways: by reducing or delaying
the disease early in the growing season or
by reducing its rate of development during
crop growth (Zadoks and Schein, 1979).
This practice has helped to block the life
cycle of the pathogens, preventing the introduction of inoculum and susceptible hosts,
eliminating certain pathogens (Palti, 1981).
Tan spot is one of a complex of necrotrophic leaf diseases of wheat which overwinter on infested crop residue (Hosford
and Busch, 1974; Loughman et al., 1998;
Carmona, 2003; Annone, 2006). The occurrence of tan spot with other leaf spots, such
as septoria blotch, spot blotch and with
rusts and mildews, can complicate disease
management practices (de Wolf et al., 1998;
Carmona, 2003; Annone, 2006). The management of tan spot is based on integrated
management of diseases that use reasonable
techniques and resources for sustainable
agriculture (Carmona, 2006).
Strategies used for the control of tan
spot are the application of fungicides, cultural control and the search for new germplasms and their incorporation in Argentina
(Carmona, 2003).
Recently in Argentina, several biological antagonists of Ptr have been identified
(Pfender et al., 1989; Li and Sutton, 1995;
Perelló et al., 2003b; Annone, 2005).
Genetic resistance
Genetic resistance is complex for diseases
such as head blight and leaf spot. The principal limitations are due to the changes
made by pathogen populations over the
years to challenge new cultivars (Carmona,
2006).
Unfortunately, only a few of the currently grown cultivars have a high level of
resistance, while somewhat larger numbers
possess a moderate level of resistance (Rees
and Platz, 1992). Kohli et al. (1992) reported
the low presence in South America of cultivars resistant to Ptr. Several studies have
been conducted in Argentina to screen
breeding material for resistance (Galich and
Galich, 1994; Annone, 1995).
In Argentina, cultivars have either a
moderate level of resistance or are susceptible to tan spot (Simón, 2006).
Tan Spot in Argentina
Chemical protection
Fungicides offer a complementary tool to
the genetic resistance available. Its use in
direct seeding crops under-compensates for
the lack of genetic protection to facultative
parasites. Fungicides are used as seed protection and/or treatment coverage with
ground or air equipment.
The majority of literature regarding the
use of fungicides to manage tan spot alone or
in combination with other leaf diseases has
focused on the timing of application and
comparative efficiencies. Research results
have been mixed, but it appears that in situations where disease pressure is high and
conditions favour further development of
foliar disease, a single well-timed application of an efficacious fungicide can reduce
disease severity, increase yield and improve
product quality (Sutton and Roke, 1986;
Bockus et al., 1992; Duczek and Jones-Flory,
1994; Stover et al., 1996).
Fungicides such as tebuconazol, frutiafol, fluzilazol propiconazol and prochloraz reduced the intensity of lesions and
showed control of from 50 to 70%, depending on the cultivar and the density of wheat
straw infested (Annone et al., 1994; Carmona, 1996). The most efficient fungicides
are systemic triazoles and estrobirulinas
(Carmona, 2003).
Cultural control
Cultural practices alter the development of
foliar diseases of wheat, particularly those
caused by facultative pathogens. Tan spot,
representative of the latter group of diseases, is affected by tillage practices in
almost all wheat-growing regions of the
world (Mehta and Gaudencio, 1991).
Retention of wheat residue on the soil
surface generally results in increased tan
spot severity (Gough and Ghazanfani, 1982;
Summerell and Burgess, 1988a,b; Schuh,
1990; Bockus and Claasen, 1992; Stover
et al., 1996; Carmona and Reis, 1998).
In areas where zero tillage is practised,
tan spot and other debris-borne diseases
283
typically increase in incidence and severity
(Rees and Platz, 1979; Mehta and Gaudencio, 1991; Kohli et al., 1992). The rotation of
crops has a high impact on the sexual stage
of this type of pathogen. Because sexual
stage viability is minor or low when wheat
straw is mineralized, the primary inoculum
is therefore low and reduces the severity of
tan spot (Carmona et al., 1999).
In relation to crop rotation, of relevance
was wheat as an antecessor of barley and
oats as an antecessor of wheat and barley as
an antecessor of oats (Carmona et al., 2001).
Barley, wheat and oats are common
hosts of Ptr and other pathogens, so there
are therefore no alternative crops available
for crop rotation. Oats were not hosts to leaf
spots specific to wheat, so crop rotation is
possible in Argentina. However, in Brazil,
Paraguay and Uruguay, where B. sorokiniana is a relevant pathogen, rotation of these
crops should not be authorized (Carmona,
2006).
Biocontrol
Biological control using antagonistic microbes alone or as supplements has become
more important in recent years in order to
minimize the use of chemicals (Annone,
2005). It is an additional tool available for
the design of more sustainable control strategies of wheat diseases. While biological
control is a widespread natural phenomenon, it is often inadequate, especially in
agricultural ecosystems in which conditions strongly favour pathogens and disease
epidemics. It can also fail in natural ecosystems, especially against aggressive alien
pathogens that are able to overcome the natural biological buffering of these systems
(Sutton, 2005).
Several biological antagonists of Ptr
have been identified (Pfender et al., 1989; Li
and Sutton, 1995; Perelló et al., 2003b; Perelló et al., 2006, 2009). Luz et al. (1998)
found that treatment with Paenibacillus
maceruns or Pseudomonas putida reduced
transmission of Ptr by seed to levels equivalent to that of a fungicide seed treatment.
284
M.V. Moreno and A.E. Perelló
Some of the fungi found to be inhibitory to
pseudothecia development by Ptr were
Limonomyces roseipellis, Myrothecium roridum, Acremoniun terricola, Stachybotrys
sp. and Laetisaria arvalis (Gough and Ghazanfani, 1982; Pfender et al., 1989). Assays
in Argentina have demonstrated that some
Trichoderma harzianum isolates are capable of suppressing growth, the mycelial
development of Ptr and the severity of diseases on wheat plants (Perelló et al., 2003b,
2006, 2008, 2009). No previous records of
antagonism between isolates of Trichoderma
spp. and the necrotrophic foliar pathogen
have been found. On the other hand, there
are increasing economic and social pressures to develop usable biological control
strategies in Argentina.
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22
Epidemiological Studies on Septoria
Leaf Blotch of Wheat in Argentina
Cristina A. Cordo
Comisión de Investigaciones Científicas de la Provincia de Buenos Aires,
Centro de Investigaciones de Fitopatología (CIDEFI) – Facultad de Ciencias
Agrarias y Forestales, La Plata, Argentina
Abstract
This chapter introduces the detailed and novel contributions on the epidemiological spread of
Mycosphaerella graminicola over the wheat field and over time. The within-season and between-crop
methods of multiplication, survival and their environmental relations are reviewed. Genetic arguments
are given to demonstrate the influence of ascospores as the major source of movement of the pathogen
into new fields. Coupled with the evidence that populations worldwide are genetically very similar, it
does seem possible that a novel form of the pathogen has been spreading worldwide. This would raise
the interesting question as to what epidemiological characteristic confers the new form’s invasiveness.
There is a clear association between the evolution of the disease and weather conditions. Wheat cultivars
exhibit differential responses to infection by M. graminicola. Breeding for disease resistance is an important tool in the integrated management of disease. Also, fungicide application and the use of biocontrol
organisms alone or in combination with fungicides is mentioned as other integrated action.
Introduction
Cereals and the processed foods derived
from them are still the principal sources of
nutrition in many parts of the world. Bread
wheat (Triticum aestivum L.) is the most
widely grown and consumed food crop. It is
the staple food of nearly 35% of the world’s
population and the demand for wheat will
grow faster than for any other major crop
(Rajaram, 1999). The forecasted global demand
for wheat in the year 2020 varies between
840 (Rosegrant et al., 1995) to 1050 Mt
(Kronstad, 1998). To meet this demand,
global production will need to increase by
1.6–2.6% annually from the present production level of 560 Mt. For wheat, the
global average yield must increase from the
current 2.5 t/ha to 3.8 t/ha. In 1995, only 18
countries worldwide had an average annual
rate of growth over 2% between 1961 and
1994 (INTA-CIMMYT, 1996). In Western
Europe and North America, the annual
growth rate for yield was 2.7% from 1977 to
1985, falling to 1.5% from 1986 to 1995
(Rajaram, 1999). Argentina, with a production of 16.11 Mt in the 2006/07 campaign
(Encuesta Agrícola, DIEA-MGAP) and
17.47 Mt in the 2007/08 campaign (OPYPA,
2008), is an important wheat exporter with
a volume of 300.00 t and an average yield of
2800 kg/ha (data from OPYPA yield and
production). The increased need for wheat
export forces producers to improve soil
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
291
292
C.A. Cordo
management and plant cultivation strategies, including the application of crop protection agents to reduce losses, and is
associated with the effective use of measures designed to increase yield and ensure
quality. With the increased proportion of
conservation tillage practice and with the
striving for optimal exploitation of the yield
and quality potential of cultivars with the
aid of appropriate cultivation and fertilization measures, the importance of certain
fungal diseases as yield-limiting factors has
increased considerably. The occurrence of
fungal pathogens can not only limit cereal
production in temperate climates, but can
also jeopardize the requisite return on capital under conditions of intensive farming.
The epidemic development of pathogens,
which vary greatly in their ecological requirements, is strongly dependent on the weather
and, together with the slightly different cultivation systems, this leads to differences in
the type of infection (pathogen species) and
in the level of infection (severity of the disease) from one year to the next. Given these
circumstances, the level of application of
fungicides in cereal cultivation in Argentina has increased since 1996. Its application produced an increased yield of 20–32%
in relation to the control test with respect to
time of application, fungicide molecular type
and wheat variety (Annone et al., 1995).
Chemical crop protection generally has
been accepted in Argentina because it
should only be used when circumstances
make it necessary to achieve the production
target and when all other options have been
considered. Crop protection measures should
be harmonized with the actual infection
situation prevailing in the crop and targeted
countermeasures must be initiated only
where there is a real risk. Cortese et al.
(1998) and Carmona et al. (1999) have fixed
the economic damage threshold (UDE) and
an action threshold (UDA) for different diseases on wheat. UDA represents the incidence value of the disease to decide the
fungicide application on the crop to reduce
the cost of application. UDE is based on the
formula of Munford and Norton (1984) and
represents the value of the disease when
the yield losses produced by the pathogen
would be equivalent to the cost of fungicide
application. The threshold values established should represent infection limits
beyond which economic losses are highly
likely in the shorter or longer term, and the
point at which the pathogen population
reach this limits in the field crop determines
the time at which fungicide should be used.
The pathogen-specific thresholds must be
worked out within the framework of exact
scientific investigations in relation to the
crop management methods used and the
environmental conditions prevailing during
the growing period. Their development
requires extensive case studies (under outdoor conditions) in order to establish how
the population dynamics and detrimental
effects are influenced by the weather, wheat
varieties, use of fertilizer, the preceding
crops and the level of inoculum. The purpose of collecting all this information is to
control individual pathogens with appropriate products at appropriate application
rates, and to do this at a time at which one
application is most likely with regard to
pathogen development and limitation of
damage, using the lowest possible input.
Epidemiology and population genetics
are different but related subsets of population biology. Epidemiology focuses on disease progression, the increase in pathogen
populations through time and the movement of pathogen populations through
space (usually from plant to plant). Most
epidemiology studies deal with a short
timescale (e.g. 1–2 growing seasons) and
small spatial scales (e.g. disease development in a field or a plantation). Epidemiology involves mainly physical processes
such as distances of spore movement or
effects of weather variables on latent periods. It does not take account of the differences in behaviour or genetically distinct
individuals in a collection of individuals.
Population genetics focuses on the processes that lead to genetic changes, or evolution, in populations over time and space.
Population genetics deals mainly with
genetic processes such as genetic drift, gene
flow, mating system, natural selection and
mutation. Present study records that epidemiological investigations, based on disease
Septoria Leaf Blotch of Wheat in Argentina
evolution, resistance supply, early detection of the disease, biological crop protection and genetic studies of the pathogen,
can be used to orientate the management of
disease under natural conditions.
The Disease as a Problem
Septoria tritici blotch (STB) is caused by M.
graminicola (Fuckel) Schroeter, in Cohn,
which is the teleomorphic stage of S. tritici
Roberger and Desmazieres (anamorph
stage). Sanderson (1972) proved the connection between the two stages and the sexual (teleomorph) form has been reported in
several countries (Hunter et al., 1999).
Cordo and Arriaga (1990) reported the sexual stage in Argentina. It is also known to
play a role in the disease’s cycle. It causes
most of the initial infection in winter wheat
crops, during the autumn in the UK (Shaw
and Royle, 1989) and USA (Schuh, 1990). In
Argentina, an increase in ascospores at harvest time has been reported, suggesting that
the sexual stage may be important in initiating the infection in the next growing season
(Cordo et al., 1999). Another possible means
of spread within a crop during summer is by
airborne ascospores, which may play a more
major role than previously recognized
(Hunter et al., 1999; Cordo et al., 2005).
Studies on the Disease’s Evolution
Several control methods, including the use
of fungicides and other cultural practices,
may reduce the effect of STB, but genetic
resistance is the most cost-effective and
environmentally safe technique to manage
the disease.
In Argentina, an inoculation technique
using oat grains covered with the stromatic
mycelia of S. tritici were presented to check
the resistance of the Septoria Monitoring
Nursery (SMN) set. The international set
created by the CIMMYT provides information on the interactions of pathogen × cultivar on different regions of the world. The use
of this set allows the generation of extensive
293
information on resistance sources for diverse
pathogen populations under variable environmental conditions. Differences among
accession reaction were significant due to
the rich composition of the selected sources
of resistance identified by CIMMYT and a
tentative group of differentials proposed by
Eyal (Gilchrist et al., 1999) that were incorporated into the SMNs.
The inoculum for grain application was
prepared in sterilized 500-ml flasks with
100 g of oat grains and 50 ml of a liquid
extract malt medium (Perelló et al., 1997).
The grains were soaked with 10 ml of an
inoculum suspension (107 conidia/ml) of S.
tritici isolate and incubated for 15–21 days
at 23 ± 2°C in darkness and shaken daily to
promote good fungal growth. After the incubation, the grains were colonized by a stromatic mycelium and were spread and dried
on trays under laboratory conditions. The
covered grains were spread on to the soil
next to the plants during the tillering growth
stage (GS23, Zadoks et al., 1974). Plants in
the plots were assessed for S. tritici infection at anthesis (GS60) and at the medium
milk (GS75) stages.
The accessions (1-BOBWHITE S; 2-TIA.
2/4/CS/TH.CU//GLEN/3/ALD/PVN;
3-CHIRYA.1; 4-CHIRYA 4; 5-CS7TH.CU//
GLEN/3/ALD/PVN/4/NANJING; 6-EG-A/H56
7.71//4#EG-A/3/2#CMH79.243; 7-MH86.540A-1Y-3B-2Y-1B-1B-1B-1Y-1M-1Y; ALD/PVN//
YMI#6; 9-SHA5/BOW; 10-ENCOY 1582–1B;
11-BOBWHITE S as the other derivative line;
12-DON ERNESTO INTA; 13-SERI M82; 14BETHLEHEM; 15-LAKHISH; 16-KAUZ; 17PENJAMO; 18-ETIT 38; 19-GLENNSON M81)
were sown in a factorial design experiment.
The pulverization inoculum was produced using the same isolate as in the previous year. The conidial concentration of the
suspension was adjusted to 1 × 107 conidia/
ml. A comparison between the pulverization and the grain application methods was
made in the field in 2000. The inoculum
suspension was sprayed on to the leaves at
the tillering stage (GS23). After inoculation,
plants were kept moist by sprinkling water
several times a day over 3 days. The severity of the infection was registered on the flag
leaf at the beginning of the flowering (GS60)
294
C.A. Cordo
and medium milk (GS75) stages using a
modified double digit Saari–Prescott scale
(Saari and Prescott, 1975). The cut for resistant behaviour was estimated as 5.3 (Gilchrist et al., 1999). Weather variables (daily
temperature, relative humidity and rainfall)
were recorded from the date of inoculation
to anthesis. Plant height was evaluated.
To compare the inoculation techniques,
both the necrotic coverage percentage (NCP)
and pycnidial coverage percentage (PCP)
were scored on the upper three leaves of 15
plants, 21 days after inoculation. The cutoff point between resistant and susceptible
response classes was 16.8% NCP following
Eyal et al. (1985). The comparison between
pulverization and grain application showed
that, except for the variety Bobwhite ‘S’ CM
33203-K-10M-7Y-3M-2Y-1M-OM and the
line Tia.2/4/CSTH.CU//GLEN/3/ALD/PVN
CIGM88.734-1B-3PR-0PR-1M which reacted
as in the observations of Gilchrist et al.
(1999), all genotypes were more susceptible
under Argentine conditions. The higher
level of virulence of the Argentine isolates
and frequency of variation could explain
this behaviour (Eyal et al., 1985; Gilchrist
et al., 1999; Cordo et al., 2006).
The results of the severity for NLP and
PCP in this study are in agreement with previous research (Eyal, 1985; Gilchrist et al.,
1999). The advanced resistant lines coming
from the crosses with a group of resistant Chinese lines did not show a high level of resistance (Ald/Pvn/YM#6, Milan/Sha#7, Catbird,
Talhuen INIA, Sha3/Seri/PSV/Bow and the
cultivar with Kavkaz/K4500 sources).
The resistant check Bethlehem was not
resistant at CIMMYT or in our conditions.
The bread wheat checks SeriM82 and
Glennson M81 (with Veery ‘S’ germplasm)
and Lakhish were susceptible, as was
expected (Gilchrist et al., 1999). The durum
wheat ETIT 38 and the resistant check Bethlehem had the same level of susceptibility
as bread wheat checks (SeriM82, Lakhish)
and as was scored by Kohli (1995). The disease resistance introduced from Brazilian
germplasm was detected on a short, earlymaturing resistant line derived from IAS 20
spring wheat and a more susceptible reaction on lines derived from IAS 58. Bobwhite
‘S’ germplasm and its derivative lines (in
Argentina represented by Don Ernesto INTA)
showed variable levels of resistance caused
by its background with more than one genetic
source and the presence of a low number of
major genes (Cordo et al., 1994).
Plant height was not associated with
the resistant reaction. The negative associations were present when weather conditions were less conducive to the development
of the disease. Non-conducive conditions
and the further distance between leaves in
tall cultivars could have reduced the rainsplash dispersal of pycnidiospores, thus
causing this negative association (Arama
et al., 1999; Simón et al., 2005; Arraiano
and Brown, 2006); it could also depend on
the presence of ascospores, which could
reduce the effect of plant height on the
expression of the disease. In Argentina, the
presence of the teleomorphic stage during
the whole growing period has been reported
(Cordo and Arriaga, 1990; Cordo et al., 1999,
2005).
The modified double digit Saari–Prescott
scale was adopted for evaluation of this set
(CIMMYT, rules for evaluation, Eyal et al.,
1987). The separate analysis of digit 1 and 2
allowed the relative height reached by the
disease to be shown simultaneously with
the severity of the damage (PCP) (Table 22.1).
The differences observed for the first digit
in the accession response to the inoculum
concentration were attributed to the maximum level of attacked leaf (8th leaf) that
was reached with the highest concentration
of inoculum: 280 g. In contrast, the second
digit did not show differences for either
concentration. The lesions were restricted in
extension, reaching only a maximum of 20%
more of the PCP in the cases of highest susceptibility. This result confirmed that the
level of resistance of tested materials was
adequate to maintain a low intensity of
infection according to the objectives proposed by Eyal and Gilchrist at the beginning
of this project (Gilchrist et al., 1999).
Two factors were influencing the expression of the disease on the leaves: the
concentration of the grain inoculum (120 g/
m2 was optimum for differentiation between
susceptible and resistant accessions) and
Septoria Leaf Blotch of Wheat in Argentina
295
Table 22.1. S. tritici infection average (digit 1 and 2) for different concentrations of inoculum and
different years.
Inoculum concentration
Accession
Digit
11
Digit
Years
21
Digit
11
Digit 2
n
C1
C2
C1
C2
1997
1999
1997
1999
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
6.00 b2
5.50 b
2.50 a
7.25 c
6.75 b
6.25 b
6.50 b
6.50 b
6.50 b
6.00 b
6.50 b
6.25 b
6.75 b
7.00 c
6.25 b
7.00 c
6.75 b
6.50 b
6.50 b
6.50 b
7.25 d
7.50 e
7.25 c
7.25 d
6.75 b
8.00 g
6.75 b
5.75 b
7.75 f
7.00 c
7.00 c
7.25 d
7.25 c
6.50 b
7.25 c
7.75 f
7.75 f
7.75 f
2.50 b
2.25 a
2.75 c
0.50 a
2.00 a
1.25 a
1.00 a
2.75 c
1.50 a
1.00 a
0.75 a
1.00 a
0.75 a
1.25 a
4.00 f
3.00 d
2.25 a
2.00 a
2.61 c
1.00 a
0.75 a
1.75 a
1.50 a
2.25 a
1.75 a
0.75 a
0.50 a
2.00 a
1.75 a
2.00 a
5.00 g
1.50 a
2.25 a
3.00 d
3.50 e
2.25 a
0.50 a
1.50 a
6.50 b2
6.75 b
5.50 a
8.00 f
7.00 c
6.75 b
7.75 e
7.00 c
6.25 b
7.25 d
7.75 e
7.25 d
7.75 e
8.00 f
6.75 b
8.00 f
8.00 f
7.750e
7.50 d
6.00 b
6.00 b
4.50 a
6.50 b
6.00 b
6.25 b
6.75 b
6.25 b
6.00 b
6.50 b
5.75 a
6.00 b
6.25 b
6.25 b
6.00 b
6.25 b
6.50 b
6.50 b
6.75 b
2.50 b
2.50 b
3.00 d
1.75 a
2.75 c
1.75 a
1.00 a
2.75 c
2.50 b
1.75 a
1.00 a
3.25 d
0.50 a
2.75 c
3.25 d
2.50 b
3.25 d
1.25 a
3.25 d
1.00 a
0.50 a
1.50 a
0.25 a
1.50 a
1.25 a
0.75 a
0.50 a
1.00 a
1.00 a
1.75 a
2.75 c
1.75 a
0.75 a
3.75 e
4.00 e
1.25 a
1.25 a
0.86 a
Note: 1As assessed by a modified double-digit Saari–Prescott scale (1975). 2Mean values followed by the same letter
are not statistically different. LSD test (P < 0.01); C1 = 120 g/m2; C2 = 280 g/m2.
the wet environment. For the grain application treatment, the density of the plants was
too important for rainfall to produce the
infection. If the rain regime was not frequent
and intensive, the pycnidiospores could not
reach the higher leaves, making it difficult
for the inoculum to ascend.
In the pulverization treatment, the surface covered by the inoculum included
more than one leaf stratum. A simultaneous
proliferation of the pathogen was obtained
in all foliage levels, which, in addition to
the beneficial structure of the canopy, produced the highest values of severity. The
most susceptible varieties at the GS75 stage
were those that had a longer period of green
leaf during the growth cycle; but something
different occurred with ETIT 38 that did not
show any difference on NCP and PCP for
both growth stages. This could be explained
by the quick senescence of the leaves that
practically stopped the development of the
fungus at the end of GS60.
The higher values of the disease in 1997
compared with those of 1999 were caused
by the influence of the climatic conditions.
The temperature was not an important factor
because there was no statistical difference in
3 years of experiments. In 1998, high humidity (30% more than the following year) and
increased rainfall (425.29 mm more than
the following year) were responsible for the
rapid increase of the disease compared with
results of 1999 (data not shown).
Both inoculation techniques were
appropriated to monitor the behaviour of
the accessions of the SMN set. If the experimental field is under a good rain regimen
from tillering to flowering, grain application
is recommended. But if it is on a dry irrigated area, pulverization with extra irrigation as a humidity chamber is suggested.
296
C.A. Cordo
Comparing the efficacy of each inoculation technique, different symptoms produced
by each treatment at the beginning of the
disease have been associated with environmental conditions (Table 22.2). In relation
to the grain application treatment, the generalized necrosis and pycnidial development on the lower leaves could have risen to
the upper leaves if irrigation or rainfall had
been present at this early stage of infection.
The density of the plants was too important
for rainfall to produce the infection. The
more compact density of the canopy helps
to maintain a microclimate for the progress
of the disease. The lack of germination that
affected seeds of some accessions could
have modified the canopy structure and the
inner microclimate; consequently, it could
have delayed the movement of the pathogen
to the upper leaves of the plant (Lovell et al.,
1997). It could explain the absence of relation between the increase of inoculum concentration and the decrease of the severity on
digit 2. In the pulverization treatment, the
surface covered by the inoculum included
more than one leaf stratum. A simultaneous
proliferation of the pathogen was obtained
in all foliage levels, which, in addition to
the beneficial structure of the canopy, produced the observed reactions.
The strong differences observed between
treatments (pulverization and grain application) could be explained by the different
rates in the progress of the disease for each
treatment. In pulverization, the inoculum
included more canopy levels. In agreement
with Lovell et al. (1997), the ascending movement of the disease was facilitated, especially
in cultivars that, because of their compressed
canopy, maintained a more favourable microclimate. On the contrary, the delayed attack
with grain application could be explained
because only the rain dispersed the conidia
from the infected grains. If the rain regimen
was not frequent and intensive, the spores
could not reach the leaves, making it difficult for the inoculum to ascend.
Although the pulverization and grain
application techniques were shown to be
effective and could be recommended for
field trials, each one demonstrated different
advantages. The grain application treatment
had the advantage that the incubation
period of the disease was maintained with
irrigation. In this case, a known weight of
inoculated grains was spread on to the soil,
between the rows, as primary inoculum. In
this technique, pycnidia on stromatic mycelia formed on grains could release pycnidiospores over a long period if wetted. This
process may be repeated several times if the
grains are dried and wetted again. The
splash dispersal effect can increase spore
transport from a low to a high level of the
crop and from plant to plant.
There was a general correlation between
the reaction of some lines and the differential varieties in relation to NCP and PCP.
The most susceptible varieties at the GS75
stage were those that had a longer period of
green leaf (8 days more); therefore, the
pathogen had a higher probability of proliferating in the leaf. Something different
occurred with Lakhish. In this differential
variety, NLC and PCP did not show any differences for both growth stages. This could
be explained by the quick senescence of the
leaves that practically stopped the development of the fungus at the end of GS60.
The technique of grain inoculation presented in this research has the advantage of
being simple to handle compared with the
installation of a barrier to infect wheat
plants artificially, as is necessary in the pulverization methods (Sanderson et al., 1986).
In the latter, it is necessary to plan the exact
date of the previous sowing and the direction of this barrier in relation to the tested
wheat rows.
Comparing infectivity of different inoculum concentrations in the grain application treatment, a gradient of infection was
obtained and its effect was related to the
variation of humidity and rain regime. The
differentiation between susceptible and
resistant entries of this set was possible
using 120 g of oat grains/m2 covered with
stromatic mycelia of S. tritici. With this, it is
not necessary to use the highest concentration of oat grains. This type of inoculum has
a long-lasting effect next to the plants, but
the most important characteristics are that the
incubation period is produced without the
installation of a wet chamber; it is simple to
Table 22.2. Necrotic and pycnidial coverage percentages caused by Septoria tritici.
Inoculum type
Pulverizationa
Nb
Pc
Grain applicationa
N
Growth Stage
a
Accessions
Pc
GS75d
N
P
51.28 cde2+
67.03 ijk
64.63 hij
54.88 efgh
52.71 def
43.67 abc
42.55 abc
61.99 ghij
88.58 m
75.84 l
45.29 bcd
60.93 fghi
43.58 abc
69.22 jkl
86.95 m
73.82 kl
74.97 l
39.23 ab
32.57 a
59.46
50.66 cd2+
67.24 f
60.00 e
37.93 a
51.37 d
36.66 a
38.15 a
51.83 d
85.49 h
70.82 fg
44.86 bc
56.90 de
43.10 b
75.26 g
82.70 h
73.48 g
65.77 f
39.30 a
41.03 ab
56.45
%
41.60 abc1+
42.59 bc
62.45 fg
27.96 a
31.92 a
46.52 bcd
38.89 ab
42.73 bcd
70.62 gh
51.60 de
48.73 cd
46.77 bcd
50.64 cde
58.97 ef
76.43 hi
83.23 i
62.21 f
66.24 fg
33.79 a
51.78
40.94 1+e
42.68 de
61.91 g
11.67 a
26.98 b
39.60 d
33.34 c
37.68 cd
67.48 g
47.35 ef
41.15 de
47.16 ef
39.80 d
51.89 f
63.16 g
83.55 h
61.88 f
65.99 g
38.58 cd
47.42
28.91 abcd1+
37.33 defg
32.26 bcde
25.32 ab
40.91 efg
18.99 a
26.14 abc
36.93 def
47.88 g
35.07 cde
45.73 fg
25.63 ab
25.32 ab
34.75 cde
44.35 fg
45.79 g
40.54 efg
19.39 a
20.97 a
33.27
28.02 def1+
37.43 i
28.17 ef
25.32 cd
37.14 hi
18.76 ab
17.97 ab
30.61 fg
48.56 j
34.66 ghi
40.59 ij
21.81 bc
20.23 abc
31.52 fgh
37.24 hi
35.10 ghi
38.19 i
14.87 a
25.48 cdf
30.09
3.21 a2+
12.89 b
30.08 bcd
1.59 a
20.12 b
21.84 b
22.48 bc
17.67 b
29.93 bcd
10.83 b
49.17 e
11.48 b
32.39 cd
24.49 bc
33.84 d
55.20 e
27.77 bcd
46.40 e
22.18 bc
25.60
18.31 cde2+
12.88 bc
30.08 fg
0.93
12.74 bc
21.70 de
13.16 bc
16.47 bcd
30.55 fg
11.18 b
36.89 gh
42.07 h
16.93 bcd
17.64 cd
17.70 cd
45.17 h
24.81 ef
41.56 e
23.02 de
21.15
Septoria Leaf Blotch of Wheat in Argentina
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Mean
GS60d
Nb
P
Note: atwo types of inoculum; bmean of necrotic coverage percentage; cmean of pycnidial coverage percentage; dtwo growth stages. 1+Each value is the average of the three upper
leaves with two inoculation methods; 2+each value is the average of the upper three leaves in two growth stages. LSD test (P < 0.01).
297
298
C.A. Cordo
transport over a long distance and to store for
a long period of time (5 days at 5°C).
Climate Influences
% Relative humidity
After the initial infection or inoculation, the
environment is one of the factors conducive
to the development of the disease. Different
experiments have demonstrated which are
the weather conditions for a more favourable expression of the disease (Simón et al.,
2005, Cordo et al., 2006). In one experiment
conducted in Argentina in 1998 (Simón
et al., 2005), the severity of the disease was
highest in the early cultivars because precipitation was higher and radiation lower for
these cultivars. Precipitation was 53.4 and
18.8 mm and radiation 3511 and 5127 Watt/m2
for a period of 15 days before evaluation for
the earliest and the latest cultivars, respectively. Also, these differences in weather
variables in 1998 produced a negative asso-
ciation between pycnidial coverage and
days to heading.
In another experiment, Cordo et al.
(2007) related that the higher values of the
disease in 1997 compared with those of 1999
were caused by the influence of the climatic
conditions from boot (GS 43) to hard ripening (GS 87) stages, following Zadoks et al.
(1974). In 1997, high humidity (30% more
than the following year) (Fig. 22.1) and
increased rainfall (425.29 mm more than
the following year) (Fig. 22.2) were responsible for the rapid increase of the disease
compared with the results of 1999. For this
experiment, temperature was not an important factor in the development of the disease
since in 3 years of experiments there were
no statistical differences in temperature
(mean temperature from the inoculation to
the end of the experiment was 17.10°C for the
first 2 years and 16.69°C for 1999). Related
to the inoculation process that is under discussion, if the experiment is to be carried
100
80
60
1997
40
1999
20
0
7Oct
14Oct
21Oct
28Oct
4Nov
11Nov
18Nov
25Nov
2Dec
Weeks
Rainfall (mm)
Fig. 22.1. Histogram showing relative humidity during October–December of 1997 and 1999.
60
50
40
30
20
10
0
1997
1999
7Oct
14Oct
21Oct
28Oct
4Nov
11Nov
18Nov
25Nov
2Dec
Weeks
Fig. 22.2. Histogram showing rainfall during October–December of 1997 and 1999.
Septoria Leaf Blotch of Wheat in Argentina
out on an artificially irrigated area, pulverization with an appropriate suspension of
spores and 48 h of extra irrigation in a wet
chamber are suggested. However, if the area
is under a good rain regime from tillering to
flowering stages, the application of grains
covered with sporulated mycelia is a feasible option.
Early Detection of the Disease
At the beginning of the wheat-growing season in 1997, Adgen Phytodiagnostic invited
the author to participate in a pilot project to
identify and quantify S. tritici and S. nodorum by antibody-based immunoassays and
also to compare with the visual method and
the sensitive methods for the early detection
of S. tritici. The objective of this work has
been to test the Adgen ELISA kit in a monitoring process on lower to higher leaves during the wheat season in Argentina.
A randomized complete block design
with four replicates and a 1.4 × 1 m size
subplot was used. Treatment consisted of
either an inoculated plot or a control treated
with a foliar fungicide spray programme.
Plantvax and Tilt were applied at 500 cm3/
ha. Ten main tillers were collected per subplot using a uniform, randomized sampling
pattern at GS10.1 (first spikelets just visible
28 October); GS10.3 (heading process 14
November); GS10.5 (flowering 27 November); GS11 (ripening 8 December). Samples
for testing consisted of ten leaves bulked for
each layer of leaves, each replication and
each data of collection. At the same time,
the severity of the lesions on the sampled
leaves was noted. Samples were homogenized in 50 ml buffer and testing following
the protocols described for the DU PONT
enzyme-linked
immunosorbent
assays
(ELISA) for S. tritici. ELISA results were
expressed as the number of S. tritici antigen
units/ml of homogeneized plant tissue
(AgU/ml). AgU/ml values were averaged.
A good correlation was observed between
ELISA readings from infected leaves coming
from different growth stages (GS10.1, GS10.3,
GS10.5) and the visual development on the
respective foliar level (C. coefficient = FL
299
0.68; FL-1 = 0.69). This indicates that the
infection increased (correlated with AgU/
ml) throughout the different growth stages
(Table 22.3). With a variance analysis, the
antigenic units registered and the severity
of the infection on three wheat cultivars
coming from inoculated and protected treatments were compared (Table 22.4). Highly
significant differences were observed between
inoculated and protected treatments for
severity and antigenic units into Flag leaf
and Flag leaf-1. A high correlation was calculated (C. coefficient = 0.56) between the average per cent visual attack of a sample and the
measured antigenic units. Despite the good
correlation, the lower interval of the level of
attack scale gave the most confirmable antigenic unit values. This immunoassay has
demonstrated to be highly sensitive and quantitative, with antigenic unit concentrations
being correlated with the severity of the disease. The infection levels of S. tritici in Los
Hornos samples could be determined with
significant precision. In addition, the specificity of the assay allowed accurate identification of this pathogen, despite the presence
of other foliar pathogens. In this assay, only
the presence of Alternaria triticimaculans
gave a cross-reaction.
The Adgen Phytodiagnostic Septoria
ELISA kit detected and quantified the
amount of S. tritici antigen in infected plant
tissues. As our experience indicates, the use
of this kit can be recommended in a monitoring process for earlier reports of S. tritici
infection.
Checking the Ascendant
Movement of the Inoculum
The ascendant movement of the inoculum
was also checked with the diagnostic immunoassay kit for S. tritici from the Adgen
Company. Increased severity on different
levels of the canopy was registered from the
latent period of the infection produced by
two types of inoculum application (grains
covered by pathogen mycelium and pulverization) during 4 weeks from inoculation at
tillering stage (GS23).
300
C.A. Cordo
Table 22.3. Correlation between percentage of lesions covered by pychnidia and antigenic units during
the 4 weeks of study.
Grain application
Susceptible
cultivars
Canopy level
Y1
B2
Y
B
Y
B
Y
B
1°a
1°a
2°b
2°b
3°c
3°c
4°d
4°d
Resistant
cultivars
1°a
1°a
2°b
2°b
3°c
3°c
4°d
4°d
Y
B
Y
B
Y
B
Y
B
Grain application
Pulverization
AU3
PCP4
AU
42 b
140 c
5.5 a
34 b
3.6 a
34 b
5.5 a
54 b
0.0 a
0.0 a
0.0 a
3.91 b
0.0 a
3.91 b
0.0 a
6.5 b
AU3
PCP4
AU
PCP
2.45 a
65 b
9.30 a
8.5 a
65 b
140 c
42 b
80 c
0.0 a
0.0 a
0.0 a
0.0 a
0.0 a
4.80 b
0.0 a
1.28 a
3.10 a
115 b
3.6 a
180 c
130 b
195 c
20 b
80 c
0.0 a
0.0 a
0.0 a
15 b
0.0 a
10 b
0.0 a
1.28 a
42 c
300 d
1.0 a
120 c
4.2 a
82 b
22 b
54 a
Pulverization
PCP
0.0 a
19.16 c
0.0 a
20.8 c
0.0 a
25 c
0.0 a
6.5 b
Note: 1youngest leaf; 2the leaf below; 3antigenic units/ml; 4pycnidial coverage percentage; a, 14 October;
b, 9 November; c, 16 November; d, 23 November.
Table 22.4.
Information summary for two populations of Septoria tritici from Argentina.
Total isolates
No. of genotypes
No. of alleles
Isolates having fingerprint data
No. of fingerprint patterns
Fingerprint pattern types
Los Hornos population
Balcarce population
58
35
24
55
14
A,E,F,G,M,N,O,P,R,S,U,V,W,X
62
39
22
58
13
A,B,D,E,H,I,K,L,M,P,Q,R,V
Six leaves per canopy level were sampled per week. Two levels of asymptomatic
leaves (the youngest and the leaf below) were
chosen from two varieties (Chirya 1 as resistant and Bethlehem as susceptible) belonging to the 8th SMN set. The first sample was
taken at GS30 (first node) stage and the following were taken one per week for 3 more
weeks. Samples were homogenized in 50 ml
buffer and tested following the protocols
described for the Adgen ELISA. An analysis
of variance was performed with the dates of
percentage of lesion covered by pycnidia
and antigen units/ml for each week. An LSD
test was used to compare treatment means.
Correlation between per cent of lesions covered by pycnidia and antigenic units were
performed throughout the treatments and
during the 4 weeks.
According to the analysis of variance
(Table 22.3), highly significant differences
were found between each level of the canopy for AU and PCP throughout the weeks
and with the two inoculation techniques.
There were significant differences for cultivars. A significant correlation was found
Septoria Leaf Blotch of Wheat in Argentina
between PCP and antigen units (C. coefficient = 0.56***) (calculated on 67 dates).
The youngest leaves almost had the lowest
values of AU. On the leaves below, it had
increased. The AU and PCP values were
higher in susceptible than in resistant cultivars. In general, on the 2nd or 3rd week
after inoculation, the infection was detected
in the youngest symptomatic leaf, indicating that the infection was installed in an
ascendant level by splashing.
Population Studies of the Pathogen
The population structure and genotypic
diversity of S. tritici from two crop field
populations in Buenos Aires Province separated by 500 km were studied with DNA
restriction fragment length polymorphism.
From of the 137 isolates from different areas
of the Argentine wheat-growing region, only
120 were characterized using the RFLP technique with P32 labelled probes. The pSTL70
fingerprinting probe hybridized many DNA
fragments of different sizes in isolates from
field populations of both locations. All leaf
samples were processed for isolation of the
fungus, followed by fungus culture, DNA
extraction, Pst1 enzyme digestion, radioactive hybridization and X-ray film detection.
Some of the isolates did not yield good quality DNA for the restriction enzyme digestion
process. This explains the loss of 17 isolates
in the samples of the populations.
In total, 24 alleles were found for the
Los Hornos population and 22 alleles for
the Balcarce population at the eight RFPL
loci (Table 22.4). Despite the difference in
the number of alleles, Nei’s measure of
genetic diversity across all loci was different for both populations (0.2619 for Los
Hornos and 0.3161 for Balcarce). Among
the 58 isolates of Los Hornos and 62 of Balcarce with complete data from individual
RFLP loci, 35 multilocus haplotypes for the
first locality and 39 for the second locality
were registered. Seven new haplotypes (3a,
20a, 71a, 37a, 47a, 52a, 58a) were added to
the list published on the Internet (S. tritici
RFLP alleles). The haplotype frequency in
301
per cent varied from 1 to 21 times for the
Los Hornos population and from 1 to 9 times
for the Balcarce population. Genotype diversity was greater in the Balcarce population
(Ĝ = 31.61 or 26.34% of the theoretical
maximum of 120) than in the Los Hornos
population (Ĝ = 26.19 or 21.82% of the theoretical maximum of 120). As the mean
genetic diversity between populations was
high for the 8 loci of RFLP, a significant difference existed between the populations of
the two localities.
Fifty-eight multilocus haplotypes and
13 fingerprint patterns were registered for
the Los Hornos population and 55 multilocus haplotypes and 14 fingerprint patterns
for the Balcarce population when they were
hybridized with pSTL70. Many isolates of
both populations had from one to several
haplotypes for each fingerprint pattern. In
the Los Hornos population, the E fingerprint
pattern was present on 14 different haplotypes, but it corresponded 3 times with the
same 11112110611 haplotype. In the Balcarce population, the same fingerprint was
present on 11 different haplotypes, but it
corresponded
8 times
with
the
11101010211 haplotype. This last result
showed that there were clones in both populations. Some genotypes were detected as
shared across the populations. In other
cases, several individuals in the two populations had the same multilocus haplotypes
but different DNA fingerprints, indicating
that they were not the same clone.
The alleles’ frequencies were significantly different from the 8 loci of RFLP. The
Argentine population must be compared
with other continental populations – Swiss
and USA (Oregon) – as independent populations. Over a total of 834 individuals,
there was a 40% gene diversity between
native populations and the total population
differentiation was 11%, showing that differentiation between native and foreign
populations exists. The average number of
migrants was 3.68. This number meant that
3–4 individuals would need to be exchanged
across populations of each generation to
maintain the observed level of genetic similarity. Moreover, the amount of gene flow
302
C.A. Cordo
between populations was high when all the
populations were compared.
The genetic distance was small when
comparing the population of Los Hornos
with the other populations, showing a high
level of similarity, but the genetic distance of
the Los Hornos and Balcarce populations was
major compared with the Oregon and Swiss
populations. Salamati et al. (2000) suggested
that the similarity among populations on a
regional basis was explained because the
gene flow was significant over spatial scales
of at least several hundred kilometres. It was
found that genetic distances among fields
within a region were small, while genetic distances among different continents were larger
for the Rhynchosporium secale populations.
Genotypic diversity within populations
and similarity over regional spatial scale was
explained because regular sexual recombination was occurring in S. tritici rather than in
R. secalis (Salamati et al., 2000), Stagonospora nodorum (McDonald et al., 1994) and
Phaeosphaeria nodorum (Keller et al., 1997)
populations. This was explained because the
ascospores from the teleomorph were dispersed over distances of up to 100 km (Shaw
and Royle, 1989; Cordo et al., 1990/1991).
The field populations of the fungus
exhibited high degrees of gene and genotype
diversity distributed on very small spatial
scales. Microgeographical-level observations
showed a higher variation of type and number of genotypes for the Balcarce than for
the Los Hornos population. In general, different genotypes were often found within a
single lesion and most lesions on the same
leaf also had different genotypes. This result
demonstrated, in coincidence with Boerger
et al. (1993), that a lesion might result due
to coinfection by two or more genotypes.
The genetic distance, for native populations, was very small considering that the
geographic distances between them was
500 km; the North American and European
populations, separated by to 7000 km, had a
low increase of this genetic distance. Then,
the high degree of similarity could be caused
by the gene flow on a regional scale and
between continents (Boerger et al., 1993;
Zhan et al 2003; Banke et al., 2004; Banke
and McDonald, 2005).
The results of this contribution are in
agreement with Keller et al. (1997), who
demonstrated that ascospores were the primary agent for unifying geographically separated populations on a regional scale.
Added to this, Cordo et al. (2005) showed
that ascospores were the most significant
component of the M. graminicola life cycle
in the wheat-producing areas in Argentina.
Their release was registered in the vegetative and debris wheat stages for the periods
analysed. According to these experiments,
the high degree of gene flow among populations would be associated neither with the
pycnidiospores presence as dominant in the
life cycle of the pathogen nor the infected
seeds that could act as a human dispersal
mechanism (Keller et al., 1997). The Los Hornos population result was different because
the clonal lineages of S. tritici probably originated from the inoculations applied for the
resistance tests.
If it is assumed that S. tritici had not
colonized Argentina recently, the high degree
of similarity could be explained from the
most likely centre of origin for this pathogen.
Banke et al. (2004) demonstrated that the
New World areas (where the South Cone is
located) appeared less likely to represent
ancestral populations because they had
lower diversity, whereas Israel and Europe
appeared to be the ancestral populations
because they showed the highest genetic
diversity. This pattern is related to the fact
that wheat has been grown in the Old World
for thousands of years, but in the New World
for only hundreds of years. Movement of
the fungus from Israel into Europe could
have been from windblown ascospores or
via transport on infected seed or straw.
Ascospore movement produced a natural
gene flow out of the possible centre of origin
and into European populations, which
could explain the finding that more haplotypes were found in European than in New
World populations.
Another way of dispersion could be an
alternate host of S. tritici producing pycnidia,
which constitute a continuous host population where ascospores (Boerger et al., 1993;
Linde et al., 2002) would maintain a uniform
source of inoculum that infects the wheat
Septoria Leaf Blotch of Wheat in Argentina
field each autumn. This way of transmission was not demonstrated in Argentina.
Disease Control with
Alternative Techniques
The most common approach to biological
control consists of selecting antagonistic
microorganisms, studying their modes of
action and developing a biological control
product. Despite progress made in the
knowledge of the modes of action of these
biological control agents, practical applications often fail to control diseases in the
field. One of the reasons for this failure is
that biocontrol products are used in the same
way as chemical products. Other methods
include the choice of an appropriate crop
rotation with the management of crop residues, added to organic amendments and
biological disinfestations of soils. In that
sense, Cordo et al. (2007) evaluated the efficacy and mechanisms of action of Trichoderma sp. for controlling leaf blotch in
wheat grown under greenhouse conditions.
Because of their capacity to act as biocontrol agents, members of the fungal genus
Trichoderma have been broadly studied (Barnett and Lilly, 1962; Tronsmo, 1986; Melo,
1991; Harman, 2000; Monte, 2001). Thus, T.
harzianum and T. aureoviride are known to
be effective antagonists against phylloplane
pathogens (Perelló et al., 1997, 2001, 2003,
2006).
There were significant differences for
necrosis and pycnidial coverage percentages
for 2 years of experiment and for the behaviour of the 14 antagonists, each treated with
303
pulverization and undercoated seed treatments. Two strains of Trichoderma sp. (Th5
and Tk11) were selected. Conversely, trials
performed during 2005 examined only plants
produced by seeds coated with Trichoderma
Th5 and Tk11 isolates. The T. koningii 11
strain was selected for the third experiment,
instead of the Th2 strain, because the necrotic
coverage percentage of Tk11 was statistically
different with respect to the control and with
a higher value than the others (Table 22.5).
Moreover, the value of pycnidial coverage
percentage was also one of the highest that
was statistically different from the control.
This work shows that T. harzianum, T.
koningi and T. aureoviride reduce the leaf
blotch caused by S. tritici in greenhousegrown wheat. The effect of T. aureoviride was
considered similar to that of T. harzianum in
reducing the leaf blotch caused by S. tritici
because, under the most effective application
method (seed coating), the pycnidial coverage
percentage was statistically different to the
control, but not to that of T. harzianum. The
positive result of the immunochemical test
applied on all asymptomatic leaf intercellular
fluid samples demonstrated the presence of
S. tritici on plants free of Trichoderma and
plants coming from Trichoderma-coated
seeds, both inoculated with S. tritici.
Effect of Trichoderma on
Leaf Proteolysis
Plants pretreated with Trichoderma Th5
and Tk11 isolates were selected to assess
the balance between leaf apoplast proteolysis and protease inhibitory capacity.
Table 22.5. Severity of necrosis and pycnidial coverage percentage in leaves with different
Trichoderma spp. isolates in 2005.
Trichoderma spp. isolates
Th5
Tk11
Control
Necrotic coverage (%)*
34.23 a
41.62 a
46.17 a
Pycnidial coverage (%)*
46.91 b
57.50 a
59.14 a
Note: *Each value is the mean of two replicates for necrotic and pycnidial coverage percentage. Means
followed by the same letter are not significantly different (P = 0.05) according to the LSD test.
304
C.A. Cordo
Compared to controls, leaf proteolytic activity decreased by 40% 12 days after S. tritici
inoculation. Conversely, it increased in
plants produced by Th5-coated seeds. This
was visible within 15–22 days after sowing
(Table 22.6). Moreover, the increased proteolytic activity coincided with a decreased
protease inhibitory capacity. Furthermore,
the proteolytic activity remained higher in
plants produced by Th5-coated seeds challenged with S. tritici. Proteolytic activity
did not increase when comparing wheat
plants grown without inoculation and
plants produced by Tk11-coated seeds.
The genus Trichoderma is a soilborne
fungus whose survival on the phylloplane
environment is difficult (Perelló et al., 1997,
2003). Thus, a Trichoderma spp. population applied over wheat leaf decreases rapidly. Conversely, coating wheat seeds with
T. harzianum is the finest application technique to control the leaf blotch caused by S.
tritici. To understand how T. harzianum acts,
the general features of the biological control
set up by Viterbo et al. (2002) were tested.
Leaves of plants formed by pre-coated seeds
did not contain T. harzianum. This suggests
that its biocontrol of leaf blotch is indirect
and able to produce morphological or biochemical changes.
As mentioned, inoculation with S. tritici decreases the apoplastic serine protease
action in plants of susceptible cultivars
(Segarra et al., 2002). Conversely, leaf blotch
symptoms decrease in susceptible cultivar
PRO INTA Molinero plants pretreated with
some Trichoderma isolates after challenging with S. tritici. For this, the protease
action in plants pretreated with isolates Th5
and Tk11 with high and low biocontrol
capacity, respectively, was tested. The apoplastic protease activity increased only after
treatment with Th5. In order to know if this
increase ocurred independently of the inoculation with the pathogenic fungus, the
kinetics of this phenomenon were analysed
in plants pretreated with this isolate. The
proteolytic activity was controlled by the
leaf germin-like protease inhibitor already
described (Segarra et al., 2003).
In plants, the apoplast forms a space
the pathogens necessarily must cross to colonize tissues. Therefore, it plays a central role
in defence strategies, being a place where not
only signals for plant response originate, but
also where the proteins for defence mechanisms accumulate: glucanases, chitinases
and proteases among others (Bowles, 1990).
Within proteases must be mentioned the
tomato P-69 induced by the citrus viroid (Vera
and Conejero, 1988; Tornero et al., 1996),
the tomato aspartil protease that degrades
proteins related to pathogenesis (Rodrigo
et al., 1988), the specific race join protease
Table 22.6. Effect of S. tritici and Trichoderma spp. on leaf apoplast proteolytic and inhibitor activity.
Treatment
Days after sowing
Protease activity (%)
Inhibitor activity (%)
T1
T2
T4 (Th5)
T4 (Th5)
T4 (Th5
T6 (Th5)
T4 (Th11)
T4 (Th11)
22
22
7
15
22
22
22
22
100
61 +/– 15
95 +/– 4
167 +/– 25
140 +/– 20
128 +/– 10
70 +/– 10
98 +/– 12
100
150
98
87
33
60
–
–
Note: In T1 (wheat plants without inoculum), the IWF (leaf intercellular washing fluid) was obtained 22 days after sowing.
In T2 (wheat plants inoculated with the pathogen) and T6 (wheat plants grown from Trichoderma spp. pre-coated wheat
seeds and inoculated with the pathogen), the plants were inoculated with S. tritici 10 days after sowing and the IWF was
examined 12 days after inoculation. In the case of plants grown from Trichoderma spp. pre-coated wheat seeds (T4), the
IWF was examined 7, 15 or 22 days after sowing. For all treatments, the proteolytic and the protease inhibition activity
was considered 100% in non-inoculated plants. Each value is the mean of two replicates.
Septoria Leaf Blotch of Wheat in Argentina
that processes the AVR9 of the compatible
reaction tomato–Cladosporium fulvum (de
Witt et al., 1985; Schaller and Ryan, 1996),
two closely related subtilisin-like proteases
that are associated with the defence response
of tomato and encoded by the P69B and
P69C genes (Jordá and Vera, 2000) and a
unique 33-kDa cysteine protease mobilized
in response to caterpillar feeding in maize
lines that are resistant to feeding by several
lepidopteran species (Pechan et al., 2002).
Conclusions
The relevant advances for Septoria leaf
blotch of wheat fall into two classes. First,
qualitative, as the conditions that allow inoculum transfer, permit infection and encourage sporulation; second, quantitative as, in
a given agroecosystem, what factors in practice control pathogen regulation size. It was
demonstrated that the distance between leaf
layers with and without infection varied
greatly according to both the architecture of
the wheat cultivar and the latent period of
the pathogen on the cultivar. The interaction of these factors, as was observed on the
SMN collection, caused great variation in
the potential for the spread of pathogens to
the upper part of the crop. DNA restriction
fragment polymorphism (RFLP) markers
labelled with radioactive compounds were
used to assess the potential for gene and
genetic diversity and for gene flow between
geographically separated populations.
The results on the genetic composition
of two populations separated by 500 km
show shared haplotypes. This has significant
implications for wheat-breeding programmes
305
that seek to incorporate resistance to S. tritici. In coincidence with Boerger et al. (1993),
our evidence of gene flow suggests that
plant breeders in Argentina are driving the
breeding process well. They are testing the
resistance of their cultivars at many locations away from the area of local adaptation.
The fine scale of patterns with genetic variability suggests that plant breeders should
use a wide spectrum of pathogen genotypes
when testing wheat cultivars resistant to
this pathogen in any location.
Throughout the genetic evidence on the
gene flow between continents, it is possible
to affirm that lesions of leaf blotch of wheat
could arise from seed transmission or could
be attributed to a failure of isolation and a
stray ascospore. In the face of the high environmental contamination produced by agrochemical products, new ecological alternatives
are applied to control diseases in extensive
plant cultures. So, biological control is a
complementary strategy in the ecological
management of wheat cultivation.
These results suggest that the saprophytic fungus T. harzianum provokes a biochemical plant defence response, as has
been reported previously. Immunochemical
tests proved that although these leaves
looked asymptomatic, they contained S.
tritici. Because T. harzianum does not meet
leaves coming from pre-coated seeds, its
stimulation of leaf proteolytic activity might
be considered a systemic induced response,
which is one of the different biochemical
mechanisms of plant defence proposed by
Viterbo et al. (2002) and Hoitink et al.
(2006). We conclude that prospects for the
biological control of leaf blotch with T. harzianum are auspicious. The results encourage trials under field conditions.
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Part VI
Alternative Control Strategies
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23
Review of Thecaphora amaranthicola
M. Piepenbr., Casual Agent of Smut on
Amaranthus mantegazzianus Pass.
M.C.I. Noelting,1 M.C. Sandoval,2 M.M.A. Gassó1 and M.C. Molina1,3
1Instituto
Fitotécnico de Santa Catalina, Facultad de Ciencas Agrarias y Forestales,
UNLP, Llavallol, Buenos Aires, Argentina; 2Facultad de Ciencas Agrarias,
UNLZ, Llavallol, Buenos Aires, Argentina; 3CONICET (Consejo Nacional de
Investigaciones Cientificas Tecnicas)
Abstract
The amaranth (Amaranthus spp.) is becoming a socially and economically important crop due to the
high level of quality proteins in its seeds and leaves. Among the factors that could limit the expansion
of this crop are the smuts (Ustilaginales) that prevent normal development of the seeds. The objectives
of the project were to: (i) characterize the pathogen which is responsible for smut on A. mantegazzianus, taking into account the cultural and morphobiometrical characters, and the germination of its
teliospores; (ii) assess the incidence of smut in two amaranth cultivars; (iii) analyse fast techniques for
the detection of the inoculum in seeds and plants cultivated in the field; and (iv) identify any possible
reservoirs of the pathogen on wild amaranths. The results obtained allowed the authors to determine
that: (i) Thecaphora amaranthicola is the causal agent of smut on A. mantegazzianus; (ii) the cultivar
Don Manuel was affected most by the smut (36% incidence); (iii) the residue and plastic card techniques are fast and efficient for the detection of the pathogen inoculum; and (iv) the wild species A.
hybridus and A. retroflexus are hosts of the pathogen. This is the first report in the world of Thecaphora amaranthicola as a pathogen of the A. mantegazzianus cultivated species.
Introduction
The amaranth is an ancestral precolumbine
crop that was cultivated by the Aztecs,
Mayas and Incas and which remained relegated for a long time after being banned by
the Spanish conquistadors. However, in the
past few decades it has been subjected to
numerous investigations with the objective
of studying its nutritional value, improvement and adaptation to new areas of cultivation (Afolabi et al., 1981; Kulakow, 1987;
Bressani, 1989; Espitia, 1991).
The seeds and leaves of this plant have
a very high level of quality proteins. This
property makes the amaranth a valuable
resource, especially appropriate for a population which lives in areas that are considered
marginal for the cultivation of traditional
cereals.
The amaranth can be affected by pests
and diseases. Among the diseases of fungal
etiology which affect the normal development of its seeds are two species of smut,
T. amaranthi (Hirschh) Vanky (syn. Glomosporium amaranthi) (Vánky, 1994) and
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
311
312
M.C.I. Noelting et al.
T. amaranthicola M. Piepenbr. (Piepembring,
2000). The spores of both types of smut
infect the ovaries, preventing normal development of the seeds. On the inside of the
affected ovaries, hypertrophied sorus development takes place and these contain a mass
of spores (teliospores). The species T. amaranthi was described and reported in wild
species of amaranth (Vánky, 1985; Hirschhorn,
1986) and in cultivated species (Alcalde,
1995; Noelting and Sandoval, 2003). On the
other hand, T. amaranthicola was described
in only one wild species of amaranth collected in Ecuador (Piepembring, 2000). In
Argentina, the data on this species are in a
report where the symptoms caused by this
smut on a cultivated species of amaranth
are described (Noelting et al., 2005a). There
are no records of T. amaranthicola on cultivated species of amaranth in the rest of the
world. Owing to the little information available on the biological and epidemiological
aspects of this smut, and that its spreading
could have a negative effect on the propagation of the amaranth crop, it was decided to
start the present study, the objectives of
which were:
1. To characterize the pathogen which is
responsible for smut on A. mantegazzianus,
taking into account the cultural and morphobiometrical characters, and the germination of its teliospores;
2. To assess the incidence of smut in two
amaranth cultivars;
3. To analyse fast techniques for the detection of the inoculum in seeds and plants; and
4. To identify any possible reservoirs of
the pathogen on wild amaranths.
Materials and Methods
Morphological and cultural
characterization of pathogen
The taxonomic identification of the fungus
was carried out by observing the teliospores,
the evolution of the germination process
(Piepembring, 2000) and the characteristics
of the cultivated colonies in vitro which
developed in potato dextrose agar (PDA 2%).
Samples of teliospores obtained from infected
seeds of amaranth (A. mantegazzianus)
were used in order to study these characters. The seeds were harvested in an experimental field of the Instituto Fitotécnico de
Santa Catalina, situated in the locality of
Llavallol, in the south of the Buenos Aires
Province, Argentina, in 2003. The teliospores were observed in an optical microscope and measured with a micrometric
objective (Carl Zeiss Jena 7×) (n = 50).
In order to observe the germination, the
teliospores were disinfected with a solution
of sodium hypochlorite (2%) for 5 min,
rinsed three times with sterile distillate
water and dried between sterile paper filters. To inoculate Petri plates, teliospores
were streaked on the surface of the agar
media to dilute the inoculum. The plates
were incubated at 25 ± 2°C with a 16 h photoperiod. To assess the colony characteristics, sections 6 mm in diameter, obtained
from a colony in active growth, were transferred to dishes with PDA. The material was
incubated in a chamber at 25 ± 2°C with a
16 h photoperiod. After 14 days of incubation, the cultural and morphological characteristics (colour, edge, diameter) of the
developed colonies were described.
Incidence assessment
The incidence assessment was carried out
in seeds of two cultivars of A. mantegazzianus (cvs. Don Juan and Don Manuel)
which had been harvested in 2004 and
presented spontaneous infections with
smut. Each panicle was harvested and
threshed by hand. Once the material had
been threshed, samples of 200 seeds/panicle were taken, determining the number
of seeds infected with T. amaranthicola.
The incidence was calculated by using the
formula:
Incidence (%) =
Number of
infected seeds
Total number of
analysed seeds
× 100
Review of Thecaphora amaranthicola M. Piepenbr.
Techniques for Detection of Inoculum
Results and Discussion
In seeds
Morphological and cultural
characterization of pathogen
Samples of four different cultivated species
of amaranth were used: A. mantegazzianus,
A. caudatus, A. hypochondriacus and A. cruentus from Argentina, Bolivia and Mexico.
The technique applied consisted in depositing 10 g of seed samples from the different
countries between pieces of reticulated cellulose paper and then submitting them to
three cycles of manual pressure. Next, the
seeds were taken out of the paper and the
residue contained in the reticules was
observed with a stereoscopic magnifying
glass (10×). The observations that tested
positive for the presence of teliospores were
confirmed by means of a preparation, with a
solution of lactophenol and cotton blue,
and observed with an optical microscope
(450×).
In plants cultivated in fields
In order to determine the infection in the
crop, 4 cm × 6 cm plastic cards were used,
with lithium grease as an adhesive on one
side of the cards. Eighteen cards were distributed randomly, 9 in each of the cultivars
of A. mantezzagianus (cvs. Don Juan and
Don Manuel), hanging from the plants for a
month. Each card was later analysed with a
stereoscopic magnifying glass to detect the
teliospores.
Wild hosts of T. amaranthicola
To detect natural reservoirs of the inoculum
of this smut, a sampling that involved three
wild species of amaranth, A. retroflexus L.,
A. hybridus L. and A. viridis L., was carried
out in 2006. The plants of these species
were located around a crop of A. mantegazzianus, as well as areas further away at a
distance of 1.5 km. The panicles of the collected material were taken to the laboratory
and observed with a stereoscopic magnifying glass and an optical microscope.
313
Morphobiometrical characteristics
of teliospores
The observations carried out in samples of
infected A. mantegazzianus seeds allowed
determination of the presence of spore balls
with the following characteristics: ochre
colour, globose to subglobose (40.78 µm ×
34.23 µm) (Fig. 23.1a). Each spore ball was
formed by 8–23 teliospores, polyhedral to
cuneiform: spore walls were deeply corrugated on the central parts of the teliospores,
as seen by SEM (Fig. 23.1b). No individual
teliospores were observed in any of the samples analysed. This phenomenon coincides
with the one observed in smuts that have
grouped spores and which develop on different kinds of plants (Barrus and Muller,
1943; Andrade et al., 2004).
The germination of the teliospores
plated on PDA was initiated after 24 h of
incubation generating phragmobasidia, followed by the development of lateral and
terminal basidiospores. Variations in the
number of germinated teliospores in each
spore ball were observed (Fig. 23.2a,b,c). In
addition to this, multiple germinations
occurred simultaneously (Fig. 23.2b,c).
The cultural characteristics of the colonies grown in laboratory conditions were as
follows: velvety surface at the expense of the
development of the mycelium, light beige
colour, slightly serrated edges and softly
lobated outline of the colony (Fig.23.2f).
The growth of the colonies was slow, reaching a maximum diameter of 34 mm after 14
days of incubation. In the colonies analysed, no yeasty type of development characteristic of this type of fungus was found.
According to analysis, T. amaranthicola,
belonging to the Basidiomycota Subkingdom, Ustilaginomycetes Class, Ustilaginomycetidae Subclass, Ustilaginales Order,
Glomosporiaceae Family, was identified as
the causal agent of smut in A. mantegazzianus (Fig. 23.3). The morphological and
cultural data shown complement the
314
M.C.I. Noelting et al.
(a)
(b)
Fig. 23.1. Teliospore balls of Thecaphora amaranthicola: (a) under the light microscope (scale bar
10 µm); (b) SEM (scale bar 10 µm).
pr
(a)
bl
(b)
(c)
Coil
(d)
(e)
(f)
Fig. 23.2. Culture of Thecaphora amaranthicola on PDA: (a) single germination of a teliospore after
24 h; pr = probasidium; (b) and (c) multiple germination of teliospores, bl = lateral basidiospores;
(d) hyaline basidiospores; (e) mycelia formation, coiling of hyphae (coil); (f) colony of T. amaranthicola
developed on PDA after 17 days of incubation.
Review of Thecaphora amaranthicola M. Piepenbr.
315
cb
(a)
(b)
(c)
Fig. 23.3. (a) Panicle of Amaranthus mantegazzianus; (b) healthy seeds (12×); (c) seeds of
A. mantegazzianus infected with T. amaranthicola (12×), seed coats show irregularities (cb).
preliminary information about the pathogen
(Noelting et al., 2005a).
Incidence
The percentage of incidence varied between
10 and 36.66% (average rates) for the Don
Juan and Don Manuel cultivars, respectively.
These results apparently indicate the existence of resistance mechanisms, especially in
the Don Juan cultivar. More studies would
have to be carried out in the future in order to
learn more about said mechanisms. Nevertheless, it cannot be discarded that the relatively
high rates of incidence which were detected
may be so because the pathogen could have
been introduced by contaminated germplasms
in the region. With respect to this, it can be
stated that the growing interest in amaranth
cultivation crop has originated the incorporation of seeds from several countries and, since
this pathology had not been reported previously in cultivated species of amaranth, there
are no controls for it in Argentina.
Inoculum detection in seed samples
Teliospores of T. amaranthicola were found
in 50% of the analysed samples (Table 23.1).
The technique applied is effective, fast and
simple (Noelting et al., 2005b) compared
with the test which consists of washing and
filtering seeds and is used for the detection
of smuts in the seeds of many crops (ISTA,
1985). The inoculum (teliospores) was
detected on and among the seeds; therefore,
individualization of the teliospores with this
technique offered information about infection
as well as contamination in amaranth seeds.
Furthermore, the retrospective character of
the analysis led to the conclusion that T. amaranthicola was already present in cultivated
species of amaranth in Argentina prior to its
first report (Noelting et al., 2005a).
In plants cultivated in the field
The employment of cards to detect aerial
inoculum in plants cultivated in the field
allowed the detection of teliospores (T.
amaranthicola) in 55% of the cards analysed, as well as other fungal propagules. This
technique is a sampling method by deposition or capture similar to those which use
slides covered by an adhesive substance
(Bugiani and Govoni, 1991). The results
obtained from both A. mantegazzianus cultivars indicate that the T. amaranthicola
teliospores are spread by the wind.
Wild host of T. amaranthicola
Seeds infected by T. amaranthicola from
panicles of A. hybridus and A. retroflexus
316
M.C.I. Noelting et al.
Table 23.1. Results of the analysis of amaranth seeds samples.
Seed sample
Locality
Province
Country
Year
Thecaphora
amaranthicola
A. cruentus cv. Don Armando
A. cruentus cv. Don Guiem
A. hypochondriacus cv. G. Covas
A. hypochondriacus Se143
A. caudatus
A. hypochondriacus cv. G. Covas
A. hypochondriacus
A. caudatus
A. hypochondriacus
A. cruentus cv. Se1MC
A. mantegazzianus cv. Don Manuel
A. mantegazzianus cv. Don Manuel
Anguil
Anguil
Anguil
Anguil
Llavallol
Luis Guillon
–
–
Llavallol
Luis Guillon
Santa Rosa
Colonia 25
de Mayo
Llavallol
Llavallol
La Pampa
La Pampa
La Pampa
La Pampa
Buenos Aires
Buenos Aires
–
–
Buenos Aires
Buenos Aires
La Pampa
La Pampa
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Mexico
Bolivia
Argentina
Argentina
Argentina
Argentina
1995
1997
1999
1999
2003
2003
2004
2004
2004
2004
2005
2005
x
x
x
x
–
–
–
–
–
x
–
–
Buenos Aires
Buenos Aires
Argentina
Argentina
2005
2005
x
x
A. mantegazzianus cv. Don Juan
A. mantegazzianus cv. Don Juan
plants were detected (Noelting et al., 2006).
The infections of a spontaneous nature
which were found in the two species of wild
amaranth that affect many crops and which
grow in a vast region of Argentina are thought
to be of epidemiological interest. This is due
to the fact that they may turn into ‘bridge’
species for the entrance of the inoculum and
the spreading of smut to cultivated amaranth
species.
Conclusions
The presence of T. amaranthicola in A. mantegazzianus (cultivated crop) is the first report,
not only in Argentina but also in the world, of
smut as a pathogen. The inoculum detection
techniques applied to samples of seeds and
plants cultivated in the field are appropriate
as a fast method for identifying the presence
of smut. Two species of wild amaranth, A.
hybridus and A. retroflexus, are hosts of T.
amaranthicola. The interest in the amaranth
crop has originated an intense interchange of
germplasms among several countries of America, Asia and Europe. This situation suggests
the need to undertake a major study of the
biological and epidemiological characteristics
of the seedborne pathogens that as the smuts
(T. amaranthi and T. amaranthicola) have
negative incidence in the crop.
References
Afolabi, A.O., Oke, O.L. and Umoh, I.B. (1981) Preliminary studies on the nutritive value of some cereal-like
grains. Nutrition Reports International 24, 389–394.
Alcalde de L, M.A. (1995) Patógenos del amaranto (Amaranthus sp.) en el sur de la provincia de Córdoba,
Argentina. Resúmenes IX Jornadas Fitosanitarias Argentinas 95.
Andrade, O., Muñoz, G., Galdames, R., Durán, P. and Honorato, R. (2004) Characterization, in vitro culture, and
molecular analysis of Thecaphora solani, the causal agent of potato smut. Phytopathology 94, 875–882.
Barrus, M.F. and Muller, S.S. (1943) An Andean disease of potato tubers. Phytopathology 33, 1086–1089.
Bressani, R. (1989) The proteins of grain amaranth. Foods Reviews International 51, 1338.
Bugiani, R. and Govoni, P. (1991) Aerobiología e difusa delle pianta. Informatore Fitopatologico 11, 9–15.
Espitia, R.E. (1991) Revancha: variedad mejorada de amaranto para los valles altos de México. In: Primer
Congreso Internacional del Amaranto. INIFAP, Chapingo, Mexico, 64 pp.
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Hirschhorn, E. (1986) Las Ustilaginales de la flora argentina. Edition CIC, La Plata, Buenos Aires, 530 pp.
International Seed Testing Association (ISTA) (1985) International rules for seed testing. Seed Science and
Technology 13, 299–513.
Kulakow, P.A. (1987) Genetics of grain amaranths. Journal of Heredity 78, 293–297.
Noelting, M.C. and Sandoval, M.C. (2003) Patógenos fúngicos en amarantos cultivados. Boletin Sociedade
Argentine de Botanica 38 (Suppl), 273.
Noelting, M.C., Sandoval, M.C. and Astiz Gassó, M.M. (2005) Primer reporte en Argentina de Thecaphora
amaranthicola como agente responsable del carbón en Amaranthus mantegazzianus (Res). Fitopatología 40, 76.
Noelting, M.C., Sandoval, M.C. and Astiz Gassó, M.M. (2005) Técnica rápida para detección de carbones
(Ustilaginales) en semillas de amaranto (Amaranthus spp.) (Res). Boletin Sociedade Argentine de
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Noelting, M.C., Astiz Gassó, M.M. and Sandoval, M.C. (2006) Especies silvestres de amarantos (Amaranthus spp.) hospedantes de Thecaphora amaranthi y T. amaranthicola (Ustilaginales) en la pcia de
Buenos Aires (Res) XII Jornadas Fitosanitarias Argentinas. Catamarca, Argentina, pp. 312–313.
Piepembring, M. (2000) New neotropical smut fungi. Mycological Research 105, 762–763.
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24
Population Biology and
Management Strategies of Phytophthora
sojae Causing Phytophthora Root
and Stem Rots of Soybean
Shuzhen Zhang1 and Allen G. Xue2
1Soybean
Research Institute, Key Laboratory of Soybean Biology of Chinese
Education Ministry, Northeast Agricultural University, Harbin, Heilongjiang, China;
2Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada,
Ottawa, Ontario, Canada
Abstract
Soybean is an important oilseed crop; it is also the richest source of protein. Root and stem rot pathogen Phytophthora sojae can reduce yield by up to 40%. Symptoms, disease cycle and genetic diversity
of P. sojae are described. The paper is a valuable document listing molecular markers and the role of
14 resistance genes located at eight genomic loci in the development of disease-resistant soybean varieties. There have been only a few fungicides available for the control of P. sojae and their effects are
limited. The development of pathogen resistance to these fungicides is not known. Little information
is available on cultural and biological controls of P. sojae. More effective management of P. sojae will
require integration of all available strategies to address all stages of the disease cycle. The integrated
approach may prove a boon to growers using a variety of susceptible cultivars. Suitable cultural,
chemical and biological methods are recommended as alternative control strategies.
Introduction
Phytophthora root and stem rot of soybean
(Glycine max (L.) Merr.), caused by P. sojae
Kaufmann and Gerdemann, is a destructive
disease throughout the soybean-planting
regions of the world (Schmitthenner, 1985).
The symptoms were first discovered as an
unknown etiology in the state of Indiana in
the USA in 1948, and subsequently in Ohio in
1951, but the causal agent was not described
until 1958 (Kaufmann and Gerdemann, 1958).
318
The disease has since been reported from
many countries, including Canada (Hildebrand, 1959), Australia (Pegg et al., 1980; Ryley
et al., 1998), Argentina and Brazil (Wrather
et al., 1997), China (Shen and Su, 1991) and
the Republic of Korea (Jee et al., 1998). The
disease is more prevalent when soil is saturated for prolonged periods of time and susceptible cultivars are planted (Grau et al.,
2004). Diseased plants reduce yield by
10–40%, or a total crop loss when infection is
severe (Anderson and Tenuta, 2003).
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Phytophthora Root and Stem Rots of Soybean
Pathogen and Disease Symptoms
P. sojae traditionally has been classified as a
fungus due to its outward resemblance of
growth habits and nutritional requirements.
In fact, it is very distant from ‘true’ fungi
evolutionarily and falls within the Kingdom
Stramenopila (Förster et al., 1990; Harper
et al., 2005), which constitutes a distinct
branch of the eukaryotic evolutionary tree
(Tyler, 2007).
The pathogen may infect soybean at
any stage of plant development and cause
seed rot, seedling pre- and post-emergence
damping-off and root and stem rots of soybean (Kittle and Gray, 1979; Athow, 1987).
The seeds could be rotted by infection of P.
sojae in both heavy and light sandy soils,
after periods of cool and rainy weather. The
damping-off, stem and root rot symptoms
often appear shortly after emergence and during early flowering when plants are under
stress (Anderson and Tenuta, 2003). The
infected seedlings have dull grey leaves and
reddish, water-soaked lesions that occur from
the base of the stem and slowly advance up
the plant, and may collapse if the infection
is severe. Symptoms on older plants are
characterized by chocolate-brown discoloration extending from the soil line to the third
or fourth node and into lower branches at
319
any time from the first trifoliate to late R9
stage (Grau et al., 2004). Severely infected
plants have few lateral roots, with almost no
nodules and only a short portion of taproots
left, and brown and drooping leaves remaining attached to the stem, even though the
plants die.
Disease Cycle
P. sojae has a narrow host range and is
restricted primarily to soybean, but there
are reports that lupin, lucerne, bean and
sweetclover could be infected using artificial inoculation in controlled environments
(Erwin and Ribeiro, 1996).
P. sojae has both asexual and sexual
stages in the life cycle and produces sporangia, zoospore and chlamydospore in the
asexual stage and oospore in the sexual
stage, as shown in Fig. 24.1 (Tyler, 2007).
Oospores are produced by the fusion of
a female organ, called oogonium, and a male
organ, antheridium. Chlamydospores are
thick-walled spores that protect the organisms surviving through periods of abiotic
stress. P. sojae can survive for many years
in soil, mainly as oospores that are formed
in the roots and stems of infected soybeans
Secondary
zoospore
Motile
zoospore
Zoosporangium
Cyst
Mycelium
Sporangium
(attached or detached)
Sexual
reproduction
Germination
Oospore
Fig. 24.1.
Phytophthora sojae life cycle.
Oogonium
Antheridium
Germinated
cyst
INFECTED
PLANT
320
S. Zhang and A.G. Xue
in large quantity and are released into the
soil when these tissues decompose (Anderson and Tenuta, 2003).
Oospores serve as the primary inoculum and germinate to produce sporangia
under flooded conditions or infective hyphae
(Anderson and Tenuta, 2003). Zoospores do
not have a cell wall and each has two flagella and are released by flooding. They can
swim a short distance (1.0 cm or less) in
saturated soil, but are disseminated primarily by moving flood water. At the end of the
motile period, which may last up to several
days, zoospore movement becomes sluggish
and jerky and encystment occurs (Schmitthenner, 2000). Zoospores can be attracted
towards the compounds excreted by soybean root tips (Morris and Ward, 1992; Tyler
et al., 1996). On reaching the root surface,
the zoospores begin to encyst and germinate,
and the hyphae penetrate directly between
the cell walls of the epidermis (BeagleRistaino and Rissler, 1983). The infection
process can be completed in 30 min in optimum conditions. On resistant cultivars,
hypersensitive response (HR) may occur, the
pathogen is contained in numerous necrotic
or dead cells and there is no development of
haustoria in the resistance interaction. However, there is no early HR reaction occurring
in susceptible cultivar; the hyphae initially
grow intracellularly and then form many
haustoria in root cells, which remain alive
in direct contact with the pathogen after
infection for around 10 h (Enkerli et al.,
1997) or approximately 12 h (Ward, 1990),
when P. sojae is able to colonize host cells
in an initial biotrophic phase of growth
without triggering any response from the
plant. After the initial infection, the pathogen begins to enter a necrotrophic growth
mode and causes many host cells to die.
The hypha penetrates from the epidermal
cells of the root into the deep layers and
vascular tissues.
Isolation and Identification of
Physiologic Races of P. sojae
P. sojae is known for the difficulty in isolation due to its slow growth. As a result, the
pathogen is often covered by bacteria and
saprophytic fungi. The commonly used
methods of isolating P. sojae are the plant
stem lesion and soil methods described by
Dorrance et al. (2008). The protocol for
plant stem lesion isolation is by surfacesterilizing cut-off tissues of symptomatic
stems first and incubating on the selective
medium, e.g. PBNIC agar, to control bacteria and other fungi, such as Pythium. P.
sojae has a distinct growth pattern on PBNIC
agar, showing white mycelium 2–3 days
after incubation. The mycelium is coenocytic and have branches almost at right
angles, with curved tips. The asexual sporangium looks like an inverted pear and the
round oospores on solid culture media can
be seen 8–10 days later. Soil isolation is
usually done by grinding the soil to fine particles, flooding it for 24 h, then draining and
air-drying the soil until it cracks or pulls
away from the side of the container, although
it is still damp. The process is to break dormancy and induce germination of oospores
in the infested soil. The soil is then used for
planting a susceptible cultivar and P. sojae
can be isolated readily from collapsed hypocotyls of emerging seedlings 5–6 days later
using the same procedure described for
plant stem lesion isolation.
The common way to store P. sojae is to
grow the pathogen on V8 juice agar slants
for 2 weeks, then cover the culture with
2 ml of sterile deionized water and store the
culture at 15°C. The fungus may be stored
in such conditions for up to 3 years without
losing its virulence and aggressiveness. For
a longer-term preservation, P. sojae can be
stored in liquid nitrogen for at least 4 years
(Dorrance et al., 2008).
Pathogenic and genetic
diversity of P. sojae
The pathogenetic variation of P. sojae was
first reported in 1958 (Kaufmann and Gerdemann, 1958). During the interaction process
with the soybean varieties, the pathogenicity and virulence of P. sojae evolved rapidly
and 55 physiologic races were identified
Phytophthora Root and Stem Rots of Soybean
based on their differential reaction on a set
of 8 or 13 differentials with a single resistance gene. Of the 55 races, races 1 to 45
were identified using a set of 8 differentials
(Bernard et al., 1957; Morgan and Hartwig,
1965; Schmitthenner, 1972; Schwenk and
Sim, 1974; Haas and Buzzel, 1976; Laviolette and Athow, 1977; Keeling, 1979, 1982;
Laviolette, 1983; White, 1983; Layton, 1986;
Wagner and Wilkinson, 1992; Henry and
Kirkpatrick, 1995; Abney et al., 1997) and
races 46 to 55 on a set of 13 differentials
(Ryley et al., 1998; Leitz et al., 2000).
Several DNA-based molecular markers
such as SSR (simple sequence repeat), rDNAITS (rDNA-internal transcribed spacer),
RFLP (restriction fragment length polymorphism) and RAPD (random amplified polymorphism DNA) have been used successfully
to identify the genetic variation and diversity of P. sojae. Whisson et al. (1992) used
RFLP to confirm sexual recombination of P.
sojae in vitro and studied the segregation of
avirulence genes. Föster et al. (1994) proposed that occasional outcrosses had been a
major contributor to the origin of new physiological races of P. sojae, in addition to
clonal evolution. Meng et al. (1999) used
the RAPD method to study populations of P.
sojae from Indiana, Iowa and Minnesota
(USA) and found no correlation of populations with a geographic origin. Wang et al.
(2003) analysed genetic diversity of 75 P.
sojae isolates from China using the RAPD
method and distinguished 12 genetic groups,
but most of the isolates were clustered into
one group and no relationship between clustering and geographic origin was found.
Wang et al. (2006) studied the genetic variation among P. sojae in the USA and China
and found that there existed higher genetic
variations in populations in the USA compared to the Chinese populations based on
the RAPD analysis. Gally et al. (2007) examined by RAPD analysis the diversity of 32 P.
sojae isolates of different geographic origins
from Argentina and detected intraspecific
variability even among isolates of the same
geographic origin. Xu et al. (2007) detected
17 P. sojae isolates from three locations in
Heilongjiang Province, China, and demonstrated by sequence analysis the difference
321
between the base constitution of ITS1 and
ITS2 among isolates. The 17 isolates were
classified into three groups based on the ITS
sequence and those isolated from the same
region belonged to the same group, which
showed the variation in geography. These
studies demonstrated that molecular tools
could be used to disclose the intraspecific
diversity of P. sojae isolates both within and
among geographic origins.
Management Strategies
The management strategies for prevention
against Phytophthora root and stem rot at
present are mainly by deployments of cultivars with race-specific or race non-specific
resistance, or a combination of the two.
Chemical methods and cultural practices
like crop rotation and tillage and integrated
management are used to a lesser extent.
Screening for resources of race-specific
and race non-specific resistance
Race-specific resistance (Rps) genes in soybean have been used extensively to manage
P. sojae (Dorrance et al., 2003). New sources
of resistance to P. sojae have been reported,
mainly from soybean varieties and germplasm in China, where soybean was originated. Lohnes et al. (1996) reported that the
Rps1d gene was common in accessions from
Anhui and Jiangsu Provinces after they
evaluated 517 soybean germplasms collected from several provinces in central
China. Kyle et al. (1998) investigated soybean accessions from southern China in
response to several races of P. sojae and
demonstrated that germplasm from Hubei,
Jiangsu and Sichuan Provinces appeared to
be valuable multi-gene resistance sources.
Lv et al. (2001) screened 956 soybean accessions from north-east China (Heilongjiang,
Jilin and Liaoning Provinces) and identified
23 varieties with resistance to both race 1
and race 25, the predominant and the most
virulent races of P. sojae in the region,
respectively. Zhang et al. (2007) evaluated
322
S. Zhang and A.G. Xue
530 soybean germplasms including 280
native soybean accessions and 250 commercial cultivars and found that the percentage
of resistance in native soybean varieties was
higher than that of the commercial cultivars. Similarly, Zhu et al. (2000, 2004), Li
et al. (2001), Huo et al. (2005) and Jin and
Zhang (2007) reported the identification of
P. sojae resistant germplasm with a number
of Rps genes from wild soybean accessions
and soybean varieties from China. In addition, Dorrance and Schmitthenner (2000)
identified several soybean accessions with
multi-gene resistance after evaluating 1015
plant introductions originated from the
Republic of Korea. These single Rps genes,
however, have often been short-lived, with
an effective ‘life’ of 8–15 years, due to the
emergence of new virulent races in response
to selection pressure exerted by the continuous use of specific resistant cultivars
(Schmitthenner and Van Doren, 1985; Ferguson, 1987; Schmitthenner et al., 1994;
Abney et al., 1997; Ryley et al., 1998).
With the known Rps genes defeated,
race non-specific resistance or partial resistance, described as the ability of plants to
survive root infection without displaying
severe disease symptoms such as death,
stunting or yield loss, is of great interest and
gains more and more attention from soybean breeders (Buzzell and Anderson, 1982;
Tooley and Grau, 1984; Schmitthenner and
Van Doren, 1985). The strategy of the combination of race non-specific resistance with
race-specific resistance is brought forward
to provide long-term management of Phytophthora root and stem rot, as well as to
avoid the boom-and-bust cycle of single
gene deployment, since it reduces the severity of root rot and slows the rate of disease
development (Buzzell and Anderson, 1982;
Burnham et al., 2003b).
Race non-specific resistance commonly
had been evaluated under natural infection
in the field until recently, owing to the
unavailability of a suitable laboratory procedure. Jimenez and Lockwood (1980) first
described a laboratory procedure for screening race non-specific resistance by growing
soybean seedlings in cups that were placed
in plastic trays containing a specified number
of zoospores of P. sojae. Irwin et al. (1982)
reported a laboratory assay by inoculating
soybean seedlings with dry P. sojae mycelium for rapid determination of relative levels of race non-specific resistance. Dorrance
et al. (2008) described a layer test and a tray
test for screening soybeans for race nonspecific resistance to P. sojae. The layer test
is done by placing inoculum of a 14-day-old
P. sojae culture 5 cm below the seeds in
cups containing coarse vermiculite and the
amount of root rot and seedling death is rated
3 weeks after planting. The tray test is
assessed by wound inoculation of a mycelial slurry on the root of 7-day-old seedlings
and root rot is rated after 7 days. Using the
layer test, Jia and James (2008) identified
several accessions with high levels of race
non-specific resistance compared with Conrade, the common known race non-specific
resistant soybean variety to P. sojae.
Resistance genes and
marker-assisted selection
A single dominant resistance gene has been
widely explored since the first resistance
gene (Rps1a) was identified by Bernard
et al. (1957). With the Rps1a defeated and
the emergence of new races in response to
selection pressure exerted by the continuous
use of Rps1a, new Rps genes are identified. A
total of 14 Rps genes including Rps1a,
Rps1b, Rps1c, Rps1d, Rps1k, Rps2, Rps3a,
Rps3b, Rps3c, Rps4, Rps5, Rps6, Rps7 and
Rps8 at eight genomic loci have been
reported so far (Bernard et al., 1957; Kilen
et al., 1974; Laviolette and Athow, 1977;
Mueller et al., 1978; Athow et al., 1980; Bernard and Cremeens, 1981; Athow and Laviolette, 1982; Ploper et al., 1985; Anderson and
Buzzell, 1992; Burnham et al., 2003a). All of
these loci have been placed on the soybean
genetic map. Rps1 and Rps3 are mapped on
molecular linkage groups (MLG) N and F,
respectively (Diers et al., 1992; Demirbas
et al., 2001; Burnham et al., 2003a). Rps2
(MLG J), Rps4 (MLG G), Rps5 (MLG G), Rps6
(MLG G), Rps7 (MLG N) and Rps8 (MLG A2)
have also been mapped (Diers et al., 1992;
Phytophthora Root and Stem Rots of Soybean
Lohnes and Schmitthenner, 1997; Demirbas
et al., 2001; Burnham et al., 2003a).
Molecular markers have been used to
facilitate selection for both single and multiresistance genes (Bent and Yu, 1999; Kumar,
1999). SSR and RFLP markers have been
identified for Rps1 (Diers et al., 1992), Rps1a
(Weng et al., 2001), Rps1b and Rps1c (Demirbas et al., 2001), Rps1d (Sugimoto et al.,
2008), Rps1k (Kasuga et al., 1997; Bhattacharyya et al., 2005), Rps2, Rps3, Rps4, Rps5,
Rps6 (Diers et al., 1992; Cregan et al., 1999)
and Rps7 (Lohnes and Schmitthenner, 1997).
With the development of the molecular biotechnique, more and more markers closer to
the resistance genes will be used in markerassisted selection (MAS), which has been
complementary to conventional breeding
programmes and to shortening the breeding
period.
In some cases, genes for complete racespecific resistance that have already been
defeated by new races of P. sojae may contribute to race non-specific resistance (Gebhardt and Valkonen, 2001), which appears
to be controlled by several genes (Walker
and Schmitthenner, 1984; Glover and Scott,
1998) and is more durable (Tooley and Grau,
1984). Several QTLs have been mapped to
linkage groups for race non-specific resistance to P. sojae. Burnham et al. (2003b)
used three recombinant inbred line (RIL)
populations with the cultivar Conrad as the
race non-specific resistance parent and
identified two putative QTLs on MLG F and
D1b + W from Conrade in all three populations. Han et al. (2008) used the RILs population of Conrade and OX760-6-1 as the race
non-specific resistance and susceptible parent, respectively, and detected three QTLs,
i.e. QGP1, QGP2 and QGP3, for Phytophthora root and stem rot tolerance. They further confirmed that QGP1 was located on
linkage group F and QGP2 in a different
interval on linkage group F and QGP3 on
linkage group D1b + W. Furthermore, an RIL
population of a cross between Conrade and
Hefeng 25 was constructed and four markers on three linkage groups, MLG D1b + W,
MLG F and MLG A2, were identified as
being associated significantly with race
non-specific resistance (Li et al., 2008).
323
Chemical, cultural, biological
and integrated control
The most commonly used chemical in the
prevention of P. sojae is metalaxyl, which is
an acylalanine fungicide specific to oomycetes. The fungicide is commonly applied
as seed treatment (apron fungicide) and infurrow, spray or granule (ridomil fungicide)
to reduce plant emergence loss and increase
yields of susceptible varieties (Anderson
and Buzzell, 1982; Guy et al., 1989).
Since soybean is the only host of P.
sojae in the field, crop rotation is an effective means of reducing the severity of the
disease. Although a short-term crop rotation
may not allow for reduction of inoculum, it
does prevent the immediate build-up of P.
sojae populations (Schmitthenner, 1985).
Because saturated soil favours the occurrence of P. sojae, tillage that could promote
soil drainage is proven to be effective in
reducing the infection period (Grau et al.,
2004). Oospores could also be buried deeper
in the soil by tillage (Workneh et al., 1998).
Biological control has been considered a
more natural and environmentally acceptable
alternative to the existing chemical treatment
methods (Cook and Baker, 1983; Baker and
Paulitz, 1996). Several bacteria and fungi have
been identified as potential bioagents in dualculture and greenhouse experiments.
There are inevitable shortcomings for
each of the prevention measurements, that
is, the Rps genes in cultivars can be defeated
by new races of P. sojae, and the varieties
with race non-specific resistance, seed treatment, rotation and tillage cannot provide an
effective control when disease pressure is
high. A combination of two or more strategies to prevent the infection of P. sojae is
very essential. The disease could be best
managed with integrated strategies in a combination of deployment of cultivars incorporated into race-specific and race non-specific
resistance genes, fungicide treatments, improved soil drainage and biocontrol. These management tactics can reduce inoculum in fields
and limit the amount of water available for
the pathogen to germinate and infect, therefore minimizing disease damage and increasing soybean production efficacy.
324
S. Zhang and A.G. Xue
Conclusions
Owing to the shift of P. sojae races, use of
race-specific resistance may quickly become
inefficient in management of the disease.
More efforts are needed in the identification
and incorporation of multi-gene resistance
and race non-specific or partial resistance
into new soybean cultivars in the future for
prevention against Phytophthora root rot
and stem rot. Soybean cultivars with a combination of two types of resistance would be
long-lived and more desirable by the soybean industry. Conventional breeding will
still be the main method of resistance breeding in the foreseeable future but, with the
rapid development of molecular technology, there will be more and more molecular
markers closely linked to the Rps genes
mapped, which could be useful in MAS to
hasten the breeding and cultivar development process.
To our knowledge, limited research has
been carried out on the expression of genes
which are possibly involved in soybean
resistance to P. sojae, notwithstanding that
14 dominant Rps genes at 8 loci have been
identified and resistance gene mapping and
quantitative trait loci have been explored.
Moy et al. (2004) reported the patterns of
gene expression on infection of soybean
plants by P. sojae race 2 and demonstrated
that genes identified as strongly upregulated
during infection included those encoding
enzymes of phytoalexin biosynthesis and
defence and pathogenesis-related proteins.
A better understanding of the resistance
mechanism in the P. sojae–soybean interaction at the molecular level is needed for
effective gene deployments and resistance
breeding. Research into these new disease
management strategies is required with transgenic soybean cultivars, in order to provide
solutions for future needs when transgenic
technologies will be more acceptable.
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25
Management of Fungal Pathogens –
A Prerequisite for Maintenance of Seed
Quality During Storage
Anuja Gupta
Indian Agricultural Research Institute, Regional Station, Karnal, Haryana, India
Abstract
Storage of seed is essential for any seed programme to sow the crop in the next season, to maintain
buffer stock as an insurance against crop failure in times of drought, excessive rainfall or other natural
calamities, to maintain parental lines for the production of hybrid seed, to conserve germplasm for
breeding purposes and for seed trade at national and international levels. Availability of good quality
seed at the right time and place is a basic prerequisite for sustaining agriculture. While maintenance
of seed germination is of utmost importance to any seed person, preservation of seed quality in terms
of its health status is equally important. A quality seed should have high genetic purity, physical
purity, seed germination, seed vigour and good health status.
Introduction
Seed health is being recognized as one of
the important criteria in evaluating seed
quality. Seed health refers primarily to the
presence or absence of disease-causing
organisms such as fungi, bacteria and viruses,
or animal pests such as nematodes and
insects, or physiological disorders due to
deficiency of trace elements. One of the
major problems associated with crop production in India is the maintenance of the
prescribed level of seed vigour and viability
from seed harvest till the next sowing season. In our plans for attaining self-sufficiency
in food grains, preventing their loss in storage is as important as the various measures
to increase production. About 60–70% of
the annual output is retained by the farmer.
Storage facilities with the farmer, as well as
with the traders, are far from satisfactory.
Farmers and traders are not fully aware of
the large savings that can be obtained by
proper storage and preservation techniques.
The important factors that determine the
longevity of seeds are seed moisture, the type
of storage container and storage environment. These factors generally interact, leading to a number of physiological and
biochemical changes in the stored seeds,
which result in deterioration of seed both in
quality and quantity, especially in tropical
and subtropical countries. According to a
current estimate, 10% of food grain is lost in
storage due to microbial spoilage and insect
attack. The damage caused by rodents and
insects is visible and therefore remedial
measures are adopted for their control, but
microbial spoilage of seeds/grains cannot be
seen easily.
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
329
330
A. Gupta
Microbial Spoilage of
Seed during Storage
Seeds are the end product of a series of steps
that include sowing, growing, harvesting
and threshing, wherein it becomes vulnerable to various pathogens/saprophytes. Among
the different microbes, fungi form a major
group of organisms that infest seeds. Nearly
150 species of fungi have been found associated with grains and seed in storage (Dharam
Vir, 1974). Mechanical damage in the seeds,
cracks, breaks or scratches in the pericarp
or seed coat developed during threshing
and processing substantially facilitate invasion by fungi, which find their way to the
storage warehouses.
The fungi found associated with seeds
during storage are known as storage fungi.
They can grow without free water, on
media with high osmotic pressure, at
RH = 70–90%. Some common storage fungi
include species of Aspergillus, Penicillium,
Rhizopus, Fusarium, Cladosporium, Alternaria, Mucor, Chaetomium, Epicoccum, etc.
Discoloration and distortion of seeds is a
major degrading factor because of seedborne
infection. Other common manifestations are
reduction in seed size, seed rots, shrivelling
of seeds, seedling decay and pre- and postemergence mortality and abnormalities. The
fungi primarily invade the embryo and in
the early stages of infection, the seed may
appear normal but, due to well-established
infections, these embryos are killed and the
seeds appear darkened. These fungi are
responsible for a decrease in market value,
germinability and nutrition of the produce,
making the grains unfit for human consumption and reducing the viability of the
seed. Excessive fungal growth may also
result in heating, caking and decay. The
seed may thus become totally spoiled, darkened or charred by prolonged exposure to
the heat generated during storage, which
brings about biochemical changes leading
to the production of toxins and loss in seed
weight.
The invasion by fungi leads to physical
and chemical changes in the seeds. Prasad
et al. (1990) observed changes in the amount
of fatty acids, glycerol, sugar and amino acids
in radish seeds infected with Aspergillus flavus. Dube et al. (1988) reported changes in
starch, fatty acids and sugars in wheat grains
infected with A. flavus and A. niger. Mishra
and Dharam Vir (1991) observed higher milling losses ranging from 34.0 to 58.6% in
discoloured rice grains. Joshi et al. (1988)
reported 73% reduction in starch content in
stored pearl millet seeds infected with storage fungi and an increase in the amount of
reducing sugars and phenolic contents. Bilgrami and Sinha (1983) have reported aflatoxin contamination in maize, groundnut
and a variety of agricultural foods and feeds.
Vaidehi (1997) showed that storage fungi
lowered the quality of maize grains due to
the biochemical changes they brought about.
Storage fungi may be present as dormant
spores or mycelium on the seed surface or
below the pericarp, which activate and multiply at a phenomenal rate under favourable
storage conditions.
Seed Mycoflora
The initial mycoflora of the seeds can give
an idea of the type of fungi that can initiate
the process of deterioration in storage.
With an increase in the storage period,
there is an increase in the incidence of
storage fungi and a decrease in seed germination (Gupta and Singh, 1990, 1993). Loss
in germination due to storage fungi may be
attributed to several factors. A toxin produced by A. ruber kills the tissues in the
embryonic axes of pea seeds in advance of
infection (Harman and Nash, 1972). Wheat
seed infected with Aspergillus spp. and
imbibed with water becomes a jelly-like
mass, suggesting that cell wall degrading
enzymes may be involved. In contrast, pea
and squash embryos are killed without
physical invasion by fungi, indicating the
involvement of diffusible toxins (Harman
and Pfleger, 1974). Mitochondria isolated
from the embryonic axes of A. ruberinfected pea seeds were less active than
those from non-infected seeds, suggesting
Management of Fungal Pathogens
that mitochondria damaged by fungi play a
role in seed deterioration (Harman and
Drury, 1973). Gupta et al. (1989) observed
a decrease in the amount of volatile aldehyde compounds with increased levels
of fungi on the seed and treatment with
benomyl increased these compounds,
indicating control of fungi and increased
germination.
An increase in seed mycoflora is correlated directly to an increase in FFA content
and leaching of solutes (especially electrolytes and water-soluble sugars) with advancing storage period (Gupta, 2003). Agarwal
(1980) demonstrated that seed deterioration
in okra, carrot and onion seeds was accompanied with leakage of sugars. Analysis of
exudates from the seeds showed that the permeability of the membrane increased with
the deterioration of seeds during ageing
(Dadlani and Agarwal, 1983). According to
Chen et al. (1998), with an increase in the
fatty acid contents of different seeds like
wheat and brassicas, storage potential decreased. Ramamoorthy and Karivaratharaju (1986)
also found that with an increase in the storage period, the oil and protein content in
groundnut seeds decreased gradually, while
free fatty acid content increased, accompanied by a loss in seed viability under ambient storage conditions. Thus, during storage,
especially under an ambient environment,
seeds produce changes due to fungal activity, resulting in deterioration of their quality (Zagrebenyer and Bern, 1998; Gupta and
Aneja, 2004).
331
Seed Mycoflora and Seed Viability
The incidence of fungal flora associated
with different seeds is low initially, but it
increases with an increase in the duration of
storage and subsequently there is a decrease
in seed viability. With the advancing storage period, the field fungi become limited
and the produce becomes infested with storage fungi. A significant negative correlation
(r = –0.793) between seed viability and seed
mycoflora has been observed with advancing storage period (Table 25.1), and consequently a decrease in seed viability.
Treatments and seed mycoflora
Seed treatments, especially with fungicides
like captan, thiram or mancozeb, restrict
the growth of mycoflora on the seeds and
maintain better seed viability (Gupta, 2003).
Moreno et al. (1985) suggested the use of
fungicides to protect the viability of corn
seeds. In another study, Moreno and Ramirez
(1983) recorded that after 330 days of storage at 26°C with 75% RH, the germination
of untreated corn seed was only 61%, while
it ranged from 68 to 90% in seeds treated
with different fungicides either singly or in
combination. The incidence of storage fungi
was also very low in treated seeds.
Kushwaha and Raut (1994) reported
that seeds treated with thiram and stored in
poly-lined bags suppressed most of the
fungi. Asalmol and Zade (1998) also observed
Table 25.1. Influence of seed mycoflora on seed viability and seed moisture during storage.
Storage period (months
after seed treatment)
Seed germination* (%)
Seed mycoflora* (%)
Seed moisture* (%)
0
2
4
6
8
10
12
Correlation coefficient (r)
92.0
86.5
92.8
85.3
50.8
44.3
38.3
–0.872
1.05
2.4
3.4
2.7
5.9
4.6
5.0
7.8
5.9
6.6
9.9
8.6
8.9
8.8
0.35
Note: *Average of 20 treatments.
332
A. Gupta
Fungal inhibition (%)
that pre-storage seed treatment helped to
improve the shelf life of seeds and checked
seed mycoflora during storage. Fungicide
seed treatments were found to restrict the
growth of mycoflora on different vegetable
seeds (Gupta and Singh, 1993).
The incidence of Colletotrichum dematium associated with chilli seed at the time
of storage was 5%. Seed treatment with captan controlled the pathogen just after its application. Other fungicides like thiride and
carbendazim controlled the pathogen after 5
months of storage, whereas in the untreated
control the pathogen persisted up to 7
months in a cloth bag and up to 15 months
in an airtight container (Gupta et al., 1992).
Thiram, bavistin and captan could control more than 96, 93 and 90% of the fungi
associated with paddy seed as against 72%
and 65% in hinosan and emisan + streptocycline treatments, respectively, after 17
months of storage under ambient conditions
(Fig. 25.1). Mancozeb (78.6%) was most effective in the control of seed mycoflora on soybean seed during storage, followed by thiram
(65.1%), bleaching powder (13.1%) and nimbecidine (10.1%) during storage (Fig. 25.2).
Onion seed variety Phule Safed dried at
6–7% moisture content and treated with
0.2% carbendazim could be stored safely in
700 gauge polyethylene bags for 32 months,
as against 24 months in untreated seed
(Mahajan et al., 2001). Pumpkin seed treated
with iodine-based halogen mixture at 3 g/kg
seed and stored in 700 gauge polythene bags
maintained seed quality. The disease, ginger
yellow, was controlled effectively by seed
treatment with 0.1% carbendazim (Rana and
Sharma, 1995).
Sandhu (1989) reported seed dressing
in the form of slurry with benlate at 1 g/kg
and dry seed treatment with brassicol at
2.5 g/kg resulted in 100% and 93.9% inhibition of germination of pea seeds after their
storage for 1 year. However, captan (0.25%)
and bayletan (0.1%) treatments enhanced
germination by 5.21 and 11.88%, respectively. He also found that in steam-sterilized
soil, germination of poor quality seeds of
pea variety Punjab-87 was enhanced from
17 to 56%. They were further enhanced up
to 69% in seeds treated with captan.
Van Toai et al. (1986) observed that only
reduced quality seed of soybean responded
to fungicide seed treatment under prolonged
storage. The accelerated ageing germination
(AAG) results of the 24-month-old methanolwashed seeds were lower than the AAG
results of the unwashed seeds (the fungicides were removed from the treated seeds
by methanol prior to the AAG test), but significantly higher for all cultivars than the
AAG values of the untreated seeds. The
fungicidal seed treatments, in addition to
protecting the seeds and seedlings during
imbibition and germination, also helped to
maintain the seed quality of soybean during
storage.
100
80
60
40
20
0
Thiram
Bavistin
Captan
Hinosan
Emisan +
streptocycline
Seed treatments
Fig. 25.1. Effect of different seed dressings on the control of seed mycoflora on paddy seed during
storage.
Management of Fungal Pathogens
Occurrence (%)
333
Inhibition (%)
Occurrence/inhibition (%)
100
80
60
40
20
Untreated
Bleaching
powder
Neembicidine
Thiram
Mancozeb
0
Seed treatments
Fig. 25.2. Influence of seed dressings on the occurrence and inhibition of seed mycoflora on soybean
seed during storage for 15 months.
Treatments and seed viability
The influence of seed treatments on seed
viability and vigour is not apparent during
the early period of storage, but becomes significant on prolonged storage. In some crops
like mung bean, mustard, muskmelon, etc.,
the effect of seed treatments on seed germination is insignificant during storage, but in
other crops like cowpea, sorghum, chillies,
fenugreek, spinach and soybean, etc., seed
treatments have a significant effect on maintaining seed viability for longer duration
under ambient storage conditions.
Mung bean cv. PS-16 retained seed dormancy for up to 6 months of storage and
viability for more than 36 months of storage
(Table 25.2). Chickpea cv. P-256 treated
with fungicides like thiram and ABC dust
retained more than 85% germination for up
to 24 months of storage. Thiram-treated
seed had better germination compared to
ABC dust treatment, and re-treatment with
thiram further enhanced seed germination.
The germination of soybean seeds, cv.
P-16, treated with mancozeb or thiram
stored in poly-lined cloth bags (polythene
bags of 400 gauge kept inside a cloth bag)
remained above the prescribed standard of
certification (70%), even on the 15th month
of storage after seed harvest, whereas germination of both treated and untreated seeds
stored in cloth bags fell below the certification standards on the 11th month of storage.
Seed treatment with mancozeb and thiram
resulted in significantly better root and
shoot lengths of seedlings as compared to
other treatments. Seedling vigour measured
in terms of seedling dry weights and/or
seedling lengths closely follow the pattern
of seed germination. The results of treating
mustard seeds with different seed dressings
are shown in Table 25.3.
The seed germination of wheat variety
HD-2329 remained above the prescribed
standard of certification (85%) for up to
20 months of storage under ambient conditions when treated with carboxin, as against
16 months in untreated seeds. The stacking
of seed bags (8 bags of 40 kg each stacked
one above the other) in the seed warehouse
had an insignificant effect on the viability
334
Table 25.2
A. Gupta
Treatments to enhance storability of legume seeds under ambient storage.
Crop seeds
Effective seed
treatment at 2 g/kg seed
Cowpea
Mung bean
Chickpea
Captan/thiram*/mancozeb
***
Thiram*
Storability in months
after seed harvest**
28
> 36
References
Gupta and Singh, 1990
Gupta and Singh, 1990
Gupta and Singh, 1990
Note: *Seed treatment at 2.5 g/kg seed; **period that seed viability remained above the prescribed standards of
certification; ***germination in treated and untreated seeds on a par.
Table 25.3. Treatments to enhance storability of oilseeds under ambient storage.
Effective treatments
Crop seeds
Seed treatment
at 2 g/kg seed
Storage container
Soybean
Mustard
Mancozeb/thiram*
***
Poly-lined bag
Poly-lined bag
Storability in months
after seed harvest**
15
24
Note: *Seed treatment at 2.5 g/kg seed; **period that seed viability remained above the prescribed standards of
certification; ***germination in treated and untreated seeds on a par.
of both treated and untreated seeds. However, varietal differences were observed
with respect to the storability of the seeds.
Wheat seed of the HD-1553 variety maintained more than 85% seed germination for
up to 15 months of storage, irrespective of
seed treatments. Germination in untreated
seeds of the HD-2285 variety fell below the
certification standard on 9 months of storage
as against seeds treated with captan, where
germination remained above 85% for up to
15 months of storage. In the HD-2009 variety,
the germination of seeds treated with carbendazim, captan or thiram was above the
prescribed standard, even after 21 months
of storage as against 15 months in untreated
seeds. The decline in germination started at
a faster rate after 21 months of storage and
reached zero level after 39 months, irrespective of variety or fungicide seed treatment
when stored under ambient conditions.
In paddy, seed germination remained
above the prescribed standard of certification (80%) for up to 20 months after seed
harvest and both the seed treatments and
storage containers had insignificant effect
on the viability of the stored seeds. The
results of fungicidal seed treatments of sorghum cv. PC-9 are shown in Table 25.4.
Among vegetable seeds, fungicidal seed
treatments also influenced germination significantly in spinach cultivar Pusa jyothi
and fenugreek (Pusa kasuri) seeds (Table
25.5). However, the effect of seed treatments
was insignificant in brinjal (Pusa kranti)
and muskmelon (Pusa madhuras) seeds.
The germination of brinjal and palak seeds
remained above the certification standards
(70% and 60%, respectively) for up to 18
months of storage after seed harvest. Spinach
seeds treated with fungicides like captan or
mancozeb had higher seed germination.
Muskmelon seeds were stored for 42 months
after seed harvest without any substantial
loss in seed viability under ambient storage
conditions. Fenugreek seeds retained viability for up to 30 months and treatment
improved seed germination as against
untreated seeds.
Fungicide treatments improved seed
germination by about 5–7% on the 30th
month of storage after seed harvest, but thereafter their influence on seed germination was
negligible. The germination of chilli seeds
Management of Fungal Pathogens
335
Table 25.4. Treatments to enhance storability of cereal seeds under ambient storage.
Crop seeds
Effective seed treatment
at 2 g/kg seed
Storability in months
after seed harvest**
Wheat
Sorghum
Paddy
Carboxin*
Captan/thiram*/brassicol/carbendazim/mancozeb
***
20
21
20
Note: *Seed treatment at 2.5 g/kg seed; **period that seed viability remained above the prescribed standards of
certification; ***germination in treated and untreated seeds on a par.
Table 25.5. Treatments to enhance storability of vegetable seeds under ambient storage.
Effective treatments
Crop seeds
Seed treatment at 2 g/kg seed
Spinach
Brinjal
Chilli
Muskmelon
Fenugreek
Mancozeb/brassicol
***
Thiram*
***
Thiram*/captan/carbendazim
Storage container
Airtight
Storability in months
after seed harvest**
18
18
19
42
30
Note: *Seed treatment at 2.5 g/kg seed; **period that seed viability remained above the prescribed standards of
certification; ***germination in treated and untreated seeds on a par.
remained above the minimum prescribed
standard (60%) for 19 months in airtight
containers as against 10 months when stored
in cloth bags, irrespective of fungicide treatments. However, thiram gave higher seed
germination.
Adverse effects of copper-oxychloride
fungicide (CuO) have been reported on vegetable seeds during storage (Gupta et al.,
1996). Fenugreek, brinjal and muskmelon
seeds treated with CuO recorded 37, 50 and
36% germination as against 69, 83 and 80%
in untreated seeds after 18, 30 and 24
months of storage, respectively. However,
in spinach seed, the CuO treatment retained
germination on a par with other seed dressings.
Paddy seeds of parental lines of paddy
IR58025A and IR58025B retained seed longevity above the prescribed standards (80%)
for up to 5 years after seed harvest when
stored under controlled conditions (temperature = 15°C; RH = 30%), as against 2 years
under ambient storage conditions in both
the parental lines (Fig. 25.3). The germination of paddy seeds of both the parental
lines stored in poly-lined bags (76.62%)
was significantly higher than seeds stored
in cloth bags (72.05%). Seed treatment with
thiram and captan also improved seed germination as against the untreated control
under both storage conditions (Fig. 25. 4).
The different treatments also influenced
the germination of parental lines of pearl
millet (MS841A, MS841B and D23) during
storage (Gupta, 2007). Seeds of MS841A,
MS841B and D23 retained germination above
minimum seed certification standards (MSCS)
(75%) for up to 16, 16 and 20 months after
seed harvest, respectively. As in paddy, the
germination of pearl millet seeds (Fig. 25.3)
stored under controlled conditions (82.25%)
was significantly higher than seeds stored
under ambient conditions (66.43%). Thus,
storage under low temperature can prolong
the longevity of the precious seeds of the
inbred parental lines of paddy and pearl
millet.
336
A. Gupta
Controlled conditions A
Controlled conditions B
Ambient conditions A
Ambient conditions B
100
Germination (%)
80
60
40
20
0
0
Fig. 25.3.
8
12
16
24
28
Storage months
36
48
60
Effect of storage conditions on seed germination in parental lines of paddy during storage.
Thiram
Captan
Untreated
100
90
Germination (%)
80
70
60
50
40
30
20
10
0
0
8
12
16
24
28
36
48
60
Storage months
Fig. 25.4.
Effect of seed treatments on seed germination in parental lines of paddy during storage.
Treatment of pearl millet seeds of different parental lines with bioagent T. viride
maintained 78.30% germination during storage that was on a par with thiram (78.34%),
captan (78.16%) and carbendazim (77.37%),
as against 74.27% in the untreated control
and 59.58% in P. fluorescence treatment (Fig.
25.6). Also, the seeds stored in polythenelined cloth bags maintained higher seed
germination in both paddy and pearl
millet than the seeds stored in cloth bags
(Fig. 25.7).
Role of Storage Environment in the
Perpetuation of Fungi
The lifespan of seed is highly influenced by
storage conditions, especially temperature
Management of Fungal Pathogens
337
Effect of storage conditions on germination in bajra seeds
Controlled storage
Ambient storage
Germination (%)
100
80
60
40
20
0
0
8
16
20
24
32
Storage months
Effect of storage conditions on seed germination in pearl millet.
Germination (%)
Fig. 25.5.
Bavistin
Captan
Thiram
Trichoderma viride
Pseudomonas fluorescence
Untreated
90
80
70
60
50
40
30
20
10
0
0
8
16
20
24
32
Storage months
Fig. 25.6.
Effect of seed dressings on seed germination in pearl millet.
and relative humidity (RH). The effects of
temperature and RH (and its subsequent effect
on seed moisture) of the storage environment
are highly interdependent. Most crop seeds
lose their viability when RH is about 80%
and the temperature varies from 25 to 30°C.
This hot and humid environment is congenial for the activity and growth of microorganisms, which leads to deterioration in seed
quality. The expression of the mycoflora
depends essentially on the temperature and
humidity conditions of seed warehouses
and also on the intergranular atmosphere of
the seed. The moisture in the seed is present
either on the seed surface or in the internal
tissues, from where it moves to the surface
and evaporates and is dependent on the RH
of the atmosphere. At different RH levels, the
equilibrium moisture content (MC) varies
with different seeds. The MC of seeds ranges
from 3 to 7, 6 to 10 and 9 to 14% at 20, 45
and 75% RH, respectively. According to Harrington’s rule of thumb (Harrington and Douglas, 1970):
●
●
For each 1% decrease in seed moisture
content, the storage life of the seed is
doubled.
For each 10°F (5.6°C) decrease in seed
storage temperature, the storage life of
the seed is doubled.
338
A. Gupta
Germination (%)
Poly-lined bag
Cloth bag
90
80
70
60
50
40
30
20
10
0
0
8
16
20
Storage months
(a)
Jute bag
24
32
Poly bag
Germination (%)
100
80
60
40
20
0
0
(b)
8
12
16
24
28
36
48
60
Storage months
Fig. 25.7. Effect of storage containers on seed germination in pearl millet (a) and paddy (b) seed
during storage.
●
The arithmetic sum of the storage temperature in degrees F and the per cent
RH should not exceed 100, with no
more than half the sum contributed by
the temperature.
However, this rule is valid only when the
seed moisture varies from 5 to 14%.
Storage fungi are unable to grow and
multiply if the MC of the stored produce is
12% or less. The optimum moisture for many
types of seed is 6–8%, at which even damage
by insects reduces (Dharam Vir, 1996). It was
observed that the per cent of seed viability
was highest at low temperatures and RH and
short storage periods, but it decreased with
increased storage period. Temperatures
above 35°C are reported to cause rapid dete-
rioration of soybean seeds in 6 months of
storage (Nkang and Umoh, 1996).
Under normal storage conditions, the
temperatures and relative humidity of the
seed warehouse fluctuates with the environment. The temperature range in a seed warehouse located in Karnal varies from 13 to
33°C and 18 to 38°C (Fig. 25.8). The relative
humidity of the seed warehouse varies from
51 to 75%. Since seed moisture is a function
of RH, so it changes with variations in the
RH of the seed warehouse. Seed dressings
do not affect the MC of seeds appreciably,
but storage containers do seem to affect it. It
is higher in seeds stored in cloth bags as
against those in poly-lined cloth bags because
cloth bags, being pervious, allow a free flow
of air from the surrounding atmosphere.
Management of Fungal Pathogens
Max. temp. (°C)
RH (%)
40
80
35
70
30
60
25
50
20
40
15
30
10
20
5
10
0
January
April
June
September
Relative humidity (%)
Temperature (°C)
Min. temp. (°C)
339
0
December
Months
Fig. 25.8.
Ambient conditions in a seed warehouse at Karnal during the year.
Although seed MC, type of storage
container and storage temperature are interrelated, high temperatures hasten the deterioration of high-moisture seeds by increasing
the metabolic activity of hydrolysed substrates and enzymes. Hence, maintaining
these factors at low levels in the seed warehouses can improve the longevity of the
seeds.
Persistence of seed
dressings during storage
Suitable dressing at proper dosage and its
uniform distribution on the seed is equally
important for proper effect of seed treatments. This also goes a long way in enhancing the storage life of seeds, especially under
ambient conditions in the seed warehouses.
Dharam Vir (1977) observed that organomercurials retained their bioefficacy for a
longer period as compared to antibiotics
and dithiocarbamates, which degrade and
become biologically ineffective after storage
of treated paddy seeds for 1 year.
The persistence of fungicides on the
seed is of paramount importance as it determines the longevity of effective seed treatment during storage. Certain fungicides,
irrespective of the amount of dressing pres-
ent on the seed, inhibit the fungi strongly,
while others exhibit weak or almost nil
inhibition. The storage container also seems
to influence the residual activity of the
chemical on treated seeds during storage.
Vyas and Nene (1971) reported minimum
loss of thiram on the seeds of cowpea, maize,
paddy and soybean in tin boxes as compared to polyethylene bags, polyethylenelined cotton bags or hessian bags. Gupta and
Chatrath (1983) found that the quantity of
thiram on soybean seeds decreased gradually with increase in the storage period and
fungicide degradation was maximum when
the seeds were stored in cloth bags, followed
by paper and alkathene-lined jute bags. Sastry and Chatrath (1984) correlated the persistence of carbendazim on wheat seeds with
storage conditions and type of container.
The quantity of fungicide on seed decreased
with the increased storage period, but the
least loss was of seed stored in polythenelined jute bags followed by polypropylene
polyethylene, cloth and jute bags, and
storage at 30°C resulted in more degradation as compared with lower temperatures.
Lakshmi and Gupta (1997) reported a significant reduction in the quantity of thiophanate methyl on soybean seed with an
extended period of storage. Maximum persistence of fungicide was found on seeds
340
A. Gupta
stored in polythene-lined jute bags, followed by polypropylene polyethylene bags.
Gupta (2002) observed a loss of 10–100% in
the activity of different chemicals during
storage. The loss was more in treated seeds
stored in cloth bag packaging compared to
treated seed stored in a polythene bag inside
a cloth bag.
The loss in the activity of the chemicals
apparently may be due partly to evaporation of the active compound of the chemical
and partly to diffusion of the compound
into the seed. Raju and Chatrath (1978)
reported that the process of the degradation
of fungicides was influenced by storage conditions. Besides, other factors such as environmental changes or physiological changes
may also be responsible for the depletion of
the activity of the chemicals. However, this
loss in the activity of the chemical can also
be correlated to the presence of seed mycoflora during storage. The loss of activity is
lower, the incidence of fungal flora is lower
and seed germination is higher.
Management of Storage
Fungi to Preserve Seed Longevity
Seed longevity can be maintained either by
reducing or preventing the fungal inoculum, or by creating unfavourable conditions
for their growth. Management strategies
should include practices both at preharvest
and postharvest stages. Proper storage and
application of safe chemicals as postharvest
treatments can control seed mycoflora effectively and reduce losses due to storage fungi
considerably. Seed treatment is one of the
most effective, safe and economic technologies which protects the seed from microbial
deterioration, and thus improves its health
status during storage and also ensures better
field emergence and seed yield.
before the crop is harvested. It is essential to
keep the crop healthy and disease free. Use
of pathogen-free/certified seed material is
the most effective method of disease-free
seed production. Disease-free seed production should be planned in safe areas and
seasons where disease development is
restricted or absent. Preharvest sprayings
with suitable chemicals, namely fungicide
or biocontrol agents, and harvesting the
crop at proper maturity also help to maintain seed quality during storage. Stress conditions during plant growth also influence
seed longevity.
Discoloration of seeds by fungi occurs
when the crop is in the field. Govindrajan
and Kannaiyan (1982) observed reduction
in grain discoloration of rice through preharvest spraying with copper oxychloride.
Seed discoloration in paddy increased with
higher levels of nitrogen and phosphorus
and decreased with larger spacing in the
field (Misra and Dharam Vir, 1992). According to Deka et al. (1996), application of
maneb at boot leaf stage, followed by spraying with common salt, was highly effective
in reducing discoloration in paddy grains.
The association of fungi is likely to be
greater in regions where the produce is harvested in the wet season. Indira and Rao
(1968) observed higher association of storage fungi in samples obtained from areas
with high humidity. Misra and Kanaujia
(1973) considered the presence of antifungal substances in the seed coat of some
oilseeds to be the reason for less storage
fungi. Nair (1982) reported fewer fungi on
seeds of Luffa acutangula because of their
thick and hard seed coat, which has a low
moisture-holding capacity. Varietal differences with regard to their susceptibility to
fungal attack during storage has been
observed by Sheeba and Ahmed (1994),
who found higher fungal incidence on seeds
of high-yielding varieties of paddy as compared to local cultivars.
Preharvest management strategies
Postharvest management strategies
For seed to remain healthy during storage,
management strategies need to be followed
from the time the crop is in the field, i.e.
Initial seed quality, seed moisture, storage
temperature and RH play an important role
Management of Fungal Pathogens
in determining seed longevity. It is essential
to avoid mechanical injuries to seed during
harvesting/threshing. The produce needs to
be dried properly to safe moisture levels
before storage. The maximum drying temperature recommended for vegetable crop
seed is 35°C. Sun drying of seeds can be
practised at the farmer’s level. The seeds are
usually packed in gunny bags or cloth bags.
Moisture-proof containers, hermetically
sealed cans, polyethylene pouches or polylined aluminium foil packets are usually
used for high-value, low-volume seeds, but
it is essential to dry the seed to 5–6% moisture level before packing in these containers
because moist seeds tend to deteriorate
faster in sealed containers in comparison to
ordinary containers.
Pre-storage seed treatment improves
the shelf life of the seed, protects it from
microbial deterioration and ensures better
seed germination and better field stand.
Many horticultural crops are propagated by
stems, roots, leaves, tubers, corms, rhizomes,
suckers, grafts and other vegetative stocks
besides seed. This propagative material may
carry several pathogens which cause different diseases, thereby affecting their field
establishment. The pathogens present in the
soil may also hamper field establishment of
these propagules. Adopting proper seed
treatment technology can reduce most of
these problems. This technology is beneficial as it involves less wastage of chemicals,
greater control over application, less environmental pollution, low risk for operators,
minimum man power, is independent of
weather conditions and has less deleterious
effects on the treated material.
Lal (1975) reported propionic acid and
potassium metabisulphite as effective against
A. niger, A. flavus, P. oxalicum and A. alternata on wheat and maize grains. Acetic acid
and propionic acid has proved effective
against A. flavus and C. lunata on groundnut
kernels. According to Vaidya and Dharam
Vir (1986, 1987), sodium metabisulphite
and propionic acid checked the growth of
Aspergillus and Penicillium sp. on groundnut kernels. After ensuring the quality of
seed material meant for storage, it is also
essential to ensure that the seed warehouses
341
where the material has to be kept is clean,
dry, cool and properly aerated. The seed
material should be packed in clean and, if
possible, new containers. If old containers
are being used, they should be disinfected
or fumigated properly to avoid any carryover pathogens.
Storage structures should not permit
entry of water by seepage from the ground
or walls. Low temperature retards the development of storage fungi on seeds and so is
advisable, especially for low-volume, highvalue seeds. It is also essential to ensure
that the seed material meant for storage
should be of high quality. The material
should be stacked on wooden pallets, maintaining a proper distance from the walls and
ceilings. The material should be checked
regularly for the development of any pests
and efficient remedial measures must be
employed immediately to keep them under
control. Thus, disease management of stored
grains requires optimum storage conditions
and deployment of treatments that do not
pose any health hazards to consumers.
Conclusions
Seed storage is highly influenced by several
intrinsic and extrinsic factors. Among them,
genotypes, seed treatment and storage containers assume a prime role in successful
seed storage. Studies are required to identify
disease-free and disease-prone areas for seed
production, as healthy seed produces healthy
crops. Certification standards for many seedborne diseases need to be developed. The
development of suitable cost-effective packaging material for safe and prolonged storage of seeds is also needed. Identification of
suitable pre-storage treatments with suitable agrochemicals and botanicals for the
seeds is another important field which
needs further work.
Some research has been initiated on an
eco-friendly approach against diseases in
the field, but their efficacy during storage
needs elucidation. The methods available
for the proper application of seed/soil dressings also need further refinement. The
application of microorganisms as agents for
342
A. Gupta
the biocontrol of plant diseases in agriculture is an important alternative to chemical
fungicides. Halogenation of seeds has also
proved a better seed storage treatment for
prolonging seed viability (Dharmalingam
et al., 2000). Invigoration treatments, namely
the effect of hydration–dehydration or invigoration with different salt solutions on
substandard seed lots for microbial growth,
need elucidation. Mid storage hydration–
dehydration treatment helps to maintain
vigour, viability and productivity of crop
seeds (Basu, 1994; Mandal et al., 2000).
Priming improves biological seed treatment,
where the primed seeds of pea and French
bean show superiority over other treatments (Rawat and Kumar, 2003). Pelleting
of seeds is also advantageous, as the application of pesticides, micronutrients, biofertilizers or plant leaf powders can be
incorporated into the seed for improvement in germination.
Some biotechnological approaches may
also be exploited by incorporating genes for
better storability of seeds. The production
of artificial seeds has unravelled new vistas in
plant biotechnology. These synthetic seeds
are artificially encapsulated propagules
used for sowing as a seed and they possess
the ability to convert into a plant under in
vitro or in vivo conditions. The preservation
of viability and vigour of somatic embryos
and synthetic seeds is one of the problems
which has to be solved prior to applying
synthetic seed technology practically. One
of the future uses of synthetic seeds would
be in germplasm conservation through
cryopreservation. With the introduction of
transgenic crops, it has become all the more
important to ensure the quality of seeds that
are traded across borders. The genetically
modified seeds need to be assessed very
carefully for any contamination with seedborne pathogens, which can be accomplished by using the modern tools of
biotechnology. Although Bt genes have
proved to be quite effective in short-term
protection against insect damage, there are
concerns that widespread use of Bt varieties
will accelerate the development of resistance to Bt in target pests. Thus, to facilitate
seed trade, strict quarantine measures at
national and international level are necessary in restricting high-risk diseases.
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26
Controlling Root and Butt Rot
Diseases in Alpine European Forests
Paolo Gonthier
Department of Exploitation and Protection of Agricultural and Forestry Resources
(DIVAPRA), Plant and Forest Pathology, University of Torino, Grugliasco, Italy
Abstract
Alpine European forests comprising of conifers and broadleaf trees at lower altitudes are facing major
problems of poor regeneration and occurrence of numerous fungal diseases. Fungal organisms like
Armillaria mellea and Heterobasidion annosum are taking their toll on a large number of conifers.
These two pathogens are responsible for most of the root and butt rot diseases in natural forest stands.
Diagnosis of disease can be done by macro/micromorphology of basidiomata. The wood-inhabiting
fungi can also be identified by taxon-specific primers using PCR. This chapter deals with various biological and cultural control strategies and the promotion of disease-tolerant plants, which can reduce
the occurrence of diseases. Integrated disease management plans are suggested for different species as
found suitable in the Aosta Valley of the western Italian Alps.
Introduction
Mountains and uplands cover approximately one-fifth of the earth’s surface and
about one-tenth of the world’s population
lives in mountain regions (Ives et al., 1997).
Mountain forests have drawn growing attention in the past few decades in both North
America and Europe. It has been hypothesized that if no forests existed in the Alps,
humans would not inhabit most of the valleys (Motta and Haudemand, 2000). Forests
protect cities and villages against avalanches, landslides, debris flows and rockfalls. They fix surface soil, prevent erosion
and play an essential role in water resource
management. They influence climate and
air quality. At present, the main functions
afforded by alpine forests are protection,
tourism and recreation, wood production,
landscape and nature conservation. Substantial economic and social changes in
mountain areas over the past few decades
have modified forest use drastically. Traditional forest functions (i.e. wood production) have been abandoned, while the
importance of other functions has grown.
Human activity has transformed mountain
forests radically in various ways: large forest areas have been destroyed and the natural composition of forests has been modified
through logging and thinning (Motta and
Haudemand, 2000).
European alpine forests comprise
mostly of conifers, while broadleaves are
generally widespread at lower altitudes,
where they may form significant stands.
Only eight native coniferous tree species are
present in the alpine region (Ozenda, 1985):
Abies alba Miller (silver fir), Picea abies (L.)
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
345
346
P. Gonthier
Karsten (Norway spruce), Larix decidua
Miller (European larch), Pinus cembra L.
(Swiss stone pine), P. sylvestris L. (Scots
pine), P. uncinata Miller (mountain pine), P.
mugo Turra (dwarf mountain pine) and P.
nigra Arnold (Austrian pine). These conifers
can grow in pure or mixed stands, depending on the site.
Most forests in the Alps are naturally
regenerated and current guidelines of forest
management are aimed at maintaining adequate levels of natural regeneration. Another
trait differentiating alpine forests from other
forests located in flat or even in mountain
areas is that in alpine forests, given their
prominent protective function, clear cut followed by artificial planting is generally forbidden. Furthermore, mixed, irregular or
uneven-aged stands are supported locally
and maintained through appropriate silvicultural practices (e.g. selective cutting,
‘forêt jardinée’) where such features fulfil
particular functions better (i.e. protection,
landscape). Several forests in the Alps, most
of which are protected forests, are affected
by major problems (Mayer, 1982), including
a lack of regeneration, a scarcity of mediumaged trees, insufficient stability and increasing vulnerability to natural disturbances.
Plant pathogens, including those causing root diseases or butt rots, may behave as
natural disturbances. In addition to causing
severe economic losses, they are reported to
influence patterns and processes in forest
ecosystems and to be affected by forest
development and landscape characteristics,
as well as by human activities (Castello et al.,
1995; Hansen and Goheen, 2000). This chapter reviews the significance and the epidemiology of the most important and widespread
root and butt rot diseases of alpine forests, as
well as the most effective and promising
control strategies to fight them.
Root and Butt Rot Pathogens,
Their Significance, Ecology and
Infection Biology
Root and butt rot fungi are important components of forest ecosystems worldwide.
Although a large number of lignicolous fungal species are reported on conifers in European mountain areas, including the Alps
(Breitenbach and Kränzlin, 1986, 1991,
1995; Bernicchia, 2005), only a few of them
are aggressive organisms having a significant impact on forests (Table 26.1). All these
fungi belong to the Basidiomycota and are
necrotrophic tree pathogens causing wood
decay. They are facultative pathogens, being
able to survive saprotrophically on dead
wood. They may be classified either as
white rot or brown rot agents depending on
the component of the plant cell wall they
are able to utilize, i.e. lignin or cellulose,
respectively.
The infection biology of wood decay
fungi in living trees has been reviewed previously (Rayner and Boddy, 1986). In general, primary infections occur by means of
airborne meiospores, which allow for the
infestation of new forest areas. Some of
these fungi may also operate a secondary,
vegetative spread, allowing for the expansion of individuals established through
primary infection. Depending on the tree
pathogen, this expansion may occur vegetatively through root grafts or contacts, leading to a tree-to-tree contagion, or by free
growth of the fungus in the soil through rhizomorphs or mycelial cords. In some pathosystems, insect vectors are essential for the
transmission of wood decay fungi (Slippers
et al., 2002).
The relative importance of primary and
secondary infection is significant not only
for our understanding of the epidemiology
and population biology of these fungi, but
also for control and management purposes.
Pathogens like Armillaria spp. and Heterobasidion spp. are able to spread secondarily.
When this happens, there is a carry-over of
the pathogen into new generations; in this
case, novel attacks are not necessarily caused
by new primary infections, but by the inoculum established at that site previously.
Most root and butt rot agents are wound
pathogens able to gain entry into the trees
through wounds or lesions. Some of them
are obligate wound pathogens (e.g. Stereum
sanguinolentum), while others are facultative
wound pathogens (e.g. Heterobasidion spp.).
Table 26.1.
Summary of characteristics of the most important root and butt rot fungi present in alpine forests.
Main hosts1
Rot type
Disease/symptoms
Infections
Reference2
Armillaria mellea
(Vahl:Fries)
P. Kummer sensu lato
Climacocystis borealis
Kotl. & Pouzar
Fomitopsis pinicola
(Sw.) P. Karst.
Heterobasidion
annosum (Fr.) Bref.
sensu lato
Several conifer and
broadleaf species
Root rot and mortality;
occasionally heart decay:
butt rot
Heart decay: butt rot
Brown, cubic
Decay: butt and stem rot
White, pocket rot
Root rot and mortality; heart
decay in roots, butt and stem
Laetiporus sulphureus
(Bull.) Murrill sensu
lato
Larch, Norway spruce;
oaks and chestnut
among broadleaves
Brown, cubic
Root contacts; active
pathogenesis through
rhizomorphs
Through wounds3 on
roots and bole
By spores through
stem wounds
By spores through fresh
stump surfaces or
wounds; root contacts
and grafts
By spores through
wounds
Guillaumin and
Legrand, 2005
Norway spruce,
silver fir, larch, pines
Silver fir, Norway
spruce, pines
Conifers; mostly
pines, Norway
spruce, silver fir
White, fibrous,
wet, often with
black zone lines
White, mottled rot
Onnia tomentosa (Fr.)
P. Karst (= Inonotus
tomentosus)
Norway spruce,
Scots pine
White, pocket rot
Phaeolus schweinitzii
(Fr.) Pat.
Stereum sanguinolentum
(Alb. & Schwein.) Fr.
Norway spruce,
silver fir, pines
Norway spruce
Brown, cubic
Heart decay: butt and stem rot;
Bernicchia, 2005;
root rot and mortality when
Butin, 2005
decay reaches the root system
involving sapwood
By spores through deep
Root rot and mortality
Tainter and
root wounds; vegetatively
Baker, 1996
through root contacts and
grafts
Heart decay: butt and stem rot
Barrett and Greig,
Vegetatively in the roots
1985; Butin, 1995
Butt and stem rot
By spores through wounds Vasiliauskas et al.,
1996; Solheim,
2006
Pale brown,
stringy rot
Solheim, 2006
Butin, 1995;
Bernicchia, 2005
Asiegbu et al., 2005
Controlling Root and Butt Rot Diseases
Fungi
Note: 1Species are listed based on their in-field susceptibility, the most susceptible listed first; 2reference for the infection biology; 3wounds are considered of mechanical origin.
347
348
P. Gonthier
Only a few root and butt rot disease agents
do not need wounds to gain entry into the
tree (e.g. Armillaria spp.). At the same time,
most root and butt rot fungi are weak, secondary pathogens, being unable to attack
vigorous trees. However, some of them are
not (i.e. Heterobasidion spp., Onnia tomentosa) and behave as primary pathogens,
which may cause significant disease with or
without pre-existing tree stresses. Weakened physiological conditions caused either
by primary or by secondary pathogens may
then trigger off attacks by other, secondary
parasites (i.e. bark beetles) (Tainter and
Baker, 1996; Jakuš, 2001).
Wood decay fungi may rot standing
trees in two ways, either starting from the
cambium and then proceeding inward (sapwood decay), or by decaying the central
portion of roots, bole and stem (heart decay)
(Rayner and Boddy, 1986). When the cambium, functional xylem or outer sapwood
are involved, several physiological functions in trees may be altered. This is particularly true for decays affecting the root
system or the collar, which generally lead to
a relatively rapid death of the host. In the
second type of decay, only the smaller
woody roots are killed, whereas the larger
ones, the bole or the stem may remain physiologically functional for a long time (Rayner
and Boddy, 1986; Gonthier et al., 2003).
Very few root and butt rot diseases have
been studied in detail, for instance those
caused by A. mellea or H. annosum species
complexes (reviewed in Shaw and Kile,
1991; Woodward et al., 1998a; Fox, 2000;
Asiegbu et al., 2005; Guillaumin, 2005).
Current knowledge on the other pathosystems here described is still very limited.
For instance, only scanty information is
available on Climacocystis borealis. This
fungus is reported as a saprophyte and secondary pathogen (Bernicchia, 2005), being
able to cause a typical heartwood rot in the
roots and the bole, which seldom reaches
more than 2–3 m in height (Solheim, 2006).
Sometimes, the sapwood is also colonized.
The borealis rot is a characteristic white
mottle rot (Bernicchia, 2005) which, on a
closer look, is cubic with white mycelium
in between (Solheim, 2006). Cubes are easily
distinguishable from those of typical cubical brown rots in that they are much finer
(1–2 mm). The fungus is rarely lethal, but
has been reported to cause significant economic losses locally and to amplify the
mechanical instability of trees during storms,
especially in mountain, mature Norway
spruce stands (Rigling et al., 2005).
Fomitopsis pinicola and Laetiporus sulphureus sensu lato are two powerful wooddestroying fungi, responsible for brown,
cubic rots. In the alpine region, the former is
associated mostly with severely damaged
silver fir and Norway spruce trees. The second species, which recently was investigated
phylogenetically (Vasaitis et al., 2009), attacks
mostly larch trees in alpine forests (Butin,
1995). They are wound pathogens, the second being able to progress towards the root
system and here producing root rot and sapwood decay. Phaeolus schweinitzii is another
widespread brown rot agent. It is reported
on all coniferous tree species growing in the
Alps and it causes a heart decay of the roots
and the bole, spreading up to 1–2 m into the
stem (Bernicchia, 2005). The infection biology of this fungus is still largely unknown.
The pathogen is believed to infect the roots
through the mycelium (Barrett and Greig,
1985; Bernicchia, 2005), which is unable to
extend freely in the soil over long distances
(Barrett and Greig, 1985). Thus, spores do
not play any primary role in the infection.
However, they are an important source of
soil infestation (Barrett, 1985). It should be
noted that other ways of infection, such as
mechanical butt wounds or root contacts
with diseased trees, have also been suggested
for this pathogen (Tainter and Baker, 1996).
There is very scanty information on the
significance of O. tomentosa in alpine forests. Its presence is probably overlooked. In
fact, despite differences in the rot type, the
fungus can be confused easily with P. schweinitzii since the basidiomata of the two
species display similar macroscopic traits
(Butin, 1995). O. tomentosa was found on
Norway spruce and Scots pine trees. This
fungus infects trees by spores through deep
root wounds and is also capable of secondary spreading through root contacts and
grafts (Tainter and Baker, 1996).
Controlling Root and Butt Rot Diseases
S. sanguinolentum is incapable of secondary spreading but is a very strong wound
colonizer, especially on Norway spruce.
Every wound, from root to top, is vulnerable
to infection, even the oldest ones (Vasiliauskas et al., 1996). Very important factors for
infection are wound size and depth (Solheim, 2006). Wound rot is initiated by injuries caused by bark-stripping red deer, as
well as by harvest-induced injuries (Cermák
et al., 2004). In Norway spruce, the potential
economic losses caused by cut-off waste
wood or low-quality logs are of considerable
magnitude and wound rot affects the trees’
stability negatively (Cermák et al., 2004).
Several taxa in the A. mellea and H.
annosum species complexes are responsible for most of the root and butt rot diseases
of conifers in natural forest stands and plantations throughout the northern temperate
regions of the world (Kile et al., 1991; Asiegbu
et al., 2005). A. mellea sensu lato encompasses about 40 biological species of varying geographic distributions, host ranges
and virulence (Pegler, 2000), seven of which
are present in Europe (Marxmüller and
Guillaumin, 2005): A. mellea (Vahl: Fries)
P. Kummer sensu stricto, A. ostoyae (Romagnesi) Herink, A. borealis Marxmüller and
Korhonen, A. gallica Marxmüller and Romagnesi, A. cepistipes Velenovský, A. tabescens
(Scopoli) Emel and A. ectypa (Fries) Lamoure. This last species is only marginally
important since it is a non-lignicolous, nonparasitic species. All the European species
may be found in the alpine area, although
some of them (i.e. A. tabescens, A. mellea),
being thermophilic, are more common at
low elevations and in the Mediterranean
region (Marxmüller and Guillaumin, 2005).
There is general agreement on the fact that
basidiospores play a marginal role in wood
colonization and infection (Guillaumin and
Legrand, 2005). Armillaria root disease may
spread either through root contacts or rhizomorphs, depending on the Armillaria species. Root contacts are essential for the
spread of A. tabescens and A. mellea, which
is characterized by fragile and short-lived rhizomorphs, while the less pathogenic A. gallica and A. cepistipes generally infect through
rhizomorphs. The ability of A. ostoyae and
349
A. borealis to produce rhizomorphs and to
infect trees in this way is variable (Guillaumin and Legrand, 2005).
In Europe, H. annosum sensu lato comprises three species, responsible for losses
estimated at more than 800m/year (Woodward et al., 1998b). H. parviporum Niemelä
& Korhonen primarily causes butt rots in
Norway spruce, but it has also been reported
to kill Scots pine saplings and attack exotics. H. abietinum Niemelä & Korhonen is
commonly associated with root or butt rots
in trees of the genus Abies, while H. annosum sensu stricto is associated typically with
root rot and mortality of trees in the genus
Pinus, but it can also be found on Picea, Juniperus and even on deciduous trees (Korhonen et al., 1998a). All the three species of
the fungus are widespread in alpine coniferous forests (Korhonen et al., 1998a; Gonthier
et al., 2001) and they are extremely pervasive locally. For instance, levels of disease
incidence of up to 95% were reported in
some subalpine Norway spruce stands in
the western Alps (Gonthier et al., 2003). In a
recent study, it was discovered that a large
majority of gaps and mortality centres in
mountain pine forests of the Swiss Alps was
caused by a Heterobasidion species rather
than by pathogenic Armillaria species (i.e.
A. ostoyae) or other factors (Bendel et al.,
2006). Heterobasidion primarily infects its
hosts by means of airborne meiospores, normally through freshly cut stumps or wounds,
and is capable of secondarily spreading
from tree to tree through root grafts and contacts (Asiegbu et al., 2005). Airborne infection through thinning stumps not only may
result in a rapid and heavy infection of a
healthy stand in areas where Heterobasidion is common (Pratt and Greig, 1988; Swedjemark and Stenlid, 1993), but also may aid
the spread of the fungus into new areas
(Berry and Dooling, 1962). Thus, stumps
play a crucial role in the epidemiology of
this forest pathogen, as confirmed indirectly
by the positive relationship between the
incidence of disease in residual trees and
the intensity of earlier thinnings, as well as
the proportion of thinning stumps infected
(Rishbeth 1957; Vollbrecht and Agestam,
1995).
350
P. Gonthier
Effects of Silviculture and
Land Management
With very few exceptions, root and butt rot
fungi are opportunistic pathogens being able
to take advantage of habitat modifications for
their establishment and spread. Thinning
and logging, as well as other forest management activities, appear to increase the damage caused by root and butt rots (Garbelotto,
2004). It has been suggested that the current
high incidence of H. annosum in unmanaged mountain pine forests of the central
Alps is due to intense logging in the past
(Bendel et al., 2006). Large amounts of timber and fuel were needed to support mining
activities, which were thriving in the area
between the 14th and 17th centuries. Similarly, the high disease severity recorded in
spruce forest stands of the western Alps
(Gonthier et al., 2003) could also be associated with mining activities in the past. In
some areas of the western Alps, intensive
cutting occurred during the 17th and 18th
centuries and this led to the creation of
stumps over large surfaces (Nicco, 1997). At
the same time, the current high level of
infestation of subalpine forests (Gonthier
et al., 2003) might also have a human origin. Cuttings have been performed regularly
at the upper edge of forests to conserve
alpine grasslands for summer grazing.
As previously stated, the creation of
stumps is particularly important for H. annosum, as stumps behave as main infection
courts for primary infections. It has been
reported that thinnings also promote the
tree-to-tree vegetative spread of the pathogen
in infected Norway spruce stands (BendzHellgren et al., 1999; Piri and Korhonen,
2008). The growth rate of the fungus in roots
increases after the felling of infected trees
(Bendz-Hellgren et al., 1999). In living spruce
roots, Heterobasidion is confined typically to
dead heartwood. Hence, the transfer of the
fungus between living trees may be limited
to functional root grafts, which enable the
fungus to grow from the xylem of infected
roots into the xylem of healthy roots. After
the tree is cut, Heterobasidion begins to
expand outwards from the centre of the root
and it may be able to spread into the surrounding trees more easily through nongrafted root contacts. Such an increase in
spreading ability after cuttings could also
occur with other heart rot fungi.
Logging operations are likely to increase
the probability of attack by most root and
butt rot fungi, since new infection courts,
i.e. wounds, are created. As an example, the
wound rot caused by S. sanguinolentum,
which is a relatively recent problem, seems
to be the result of increasing mechanization
of forestry (Butin, 1995). With the use of
heavy machinery for the extraction of thinnings, large bark wounds occur much more
frequently. This may be crucial for the
establishment of fungi, like S. sanguinolentum, that need wounds larger than
10 × 10 cm to infect trees (Butin, 1995). In
general, wounds play an important or fundamental role in the infection biology of fungi
that are able to produce infective airborne
inoculum. Nevertheless, it has been proposed
that wounds could also trigger attacks by
fungi unable to infect by spores, i.e. Armillaria spp., because they can induce a lowering
of tree defences (Popoola and Fox, 1996).
Diagnosis and General
Control Strategies
Management options are based on our knowledge of the ecology, epidemiology and infection biology of the causal agent. Hence,
before planning control, there is good reason to perform an accurate diagnosis and
identification of the causal agent.
The type of rot may aid in the diagnosis
but in general it is not an exhaustive trait for
the identification of wood decay fungi (Bernicchia, 2005; Solheim, 2006). Traditionally, diagnosis is based on the macro- and/
or micromorphology of basidiomata and it
may be achieved through the use of mycological keys (Eriksson et al., 1984; Breitenbach
and Kränzlin, 1986, 1991, 1995; Bernicchia,
2005; Gonthier and Nicolotti, 2007). When
performing field diagnosis, a pathologist
should consider that basidiomata of wood
decay fungi usually emerge at advanced
Controlling Root and Butt Rot Diseases
stages of the fungal infection and they may
be rarely or sporadically visible. Furthermore, basidiomata of some species (e.g. P.
schweinitzii) are short-lived (Butin, 1995)
or may be found only during particular periods of the year (i.e. Armillaria spp.); thus,
the timing of diagnosis is also important.
Most root and butt rot fungi can be cultured
easily from decayed wood. A few of them
(i.e. Heterobasidion spp., L. sulphureus)
develop a fast-growing conidial stage in culture or when colonized wood is incubated in
a damp room. Usually, asexual mitospores of
these fungi do not play any significant role
in the infection biology. Nevertheless, they
may have a diagnostic value. Identification
of wood-inhabiting fungi, non-sporulating
in pure culture is also possible by using
appropriate keys (Nobles, 1965; Stalpers,
1978). Pure culture analysis, however, is
difficult and time-consuming.
A number of molecular techniques have
been developed and are now available for
351
the identification of the most important root
and butt rot fungi. A summary of them is
given in Table 26.2. Some techniques allow
for the identification of rots directly from
wood. For instance, polymerase chain reaction (PCR) with taxon-specific primers provides reliable fungal diagnostics from both
pure culture and environmental samples.
Theoretically, a pathogen can be controlled
during all stages of its life cycle, starting from
primary infection and early establishment,
through spreading inside the host, to formation, spread and survival of its propagules
(Holdenrieder and Greig, 1998). Unfortunately, root and butt rot diseases are virtually
impossible to eradicate once they are established. They may be controlled successfully
only when pathogens have a small biomass
and are therefore weakly competitive.
In general, when dealing with root and
butt rot diseases characterized by abundant
primary infection events (i.e. H. annosum,
S. sanguinolentum, etc.), forest management
Table 26.2. Taxon-specific primers developed for the identification of some of the most important root
and butt rot fungi present in alpine forests.
Fungi
Forward primer
Reverse primer
Amplicon size
Reference
A. mellea
sensu lato1
ARM-1 (agggta
tgtgcacgttcgac)
ITS3 (gcatcgat
gaagaacgcagc)
HET-7 (cttctcac
aaactcttcg)
MJ-F (ggtcctgtc
tggctttgc)
KJ-F (ccattaac
ggaaccgacgtg)
MLF (taaaaatttaa
attagccataa)
ARM-2 (ggaaagctaa
gctcgcgcta)
Armi2R (aaacccccat
aatccaatcc)
HET-8 (caggtccccca
caatcg)
MJ-R (ctgaagcacac
cttgcca)
KJ-R (gtgcggctcattc
tacgctatc)
Mito7 (gccaatttatttt
gctacc)
Mito5 (taagaccgctata
waccagac)
660 bp
Schulze et al., 1997
184 bp
Guglielmo et al., 2007
400 bp
Bahnweg et al., 2002
100 bp
Hantula and
Vainio, 2003
H. annosum
sensu lato
H. annosum
sensu lato
H. parviporum
H. annosum
sensu stricto
H. abietinum
H. parviporum
L. sulphureus
sensu lato
O. tomentosa
Stereum spp.
MLS (aaattagcca
tattttaaaag)
25sF (tggcgaga
LaetR (ccgagcaaac
gaccgatagc)
gaatgcaa)
It-ITS-209-f (gcta
It-ITS-700-rc (agga
aatccactcttaacac)
gccgaccacaaaagat)
ITS3 (gcatcgatg
Ste2R (gtcgcaacaa
aagaacgcagc)
gacgcactaa)
350 bp
230 bp
195 bp
185 bp
Garbelotto et al.,
1998; Gonthier et al.,
2001, 2003
146 bp
Guglielmo et al., 2007
491 bp
Germain et al., 2002
234–240 bp
Guglielmo et al., 2007
Note: 1Some Armillaria species may be distinguished through PCR-restriction fragment length polymorphism (RFLP)
(Harrington and Wingfield, 1995; Schulze et al., 1997; Sierra et al., 1999).
352
P. Gonthier
should focus on minimizing those activities
likely to create good primary infection courts,
e.g. wounds on roots, stems and stumps. Care
should be taken in order to avoid harvestinduced injuries. Containment of wildlife
populations, especially of bark-stripping
deers, may also have some effects on wound
rot severity (Cermák et al., 2004).
Lowering stand density may regulate
secondary infection events, especially for
pathogens spreading through root grafts and
contacts. In addition, a regulation of stand
density aimed at reducing tree competition
may prevent or reduce infections by a wide
range of weakness pathogens, regardless of
their mode of transmission (i.e. Armillaria
spp., F. pinicola, etc.).
The management of mature alpine forests that are diseased or under the risk of
root and butt rots could be achieved through
several other and more specific methods,
which are reviewed below. Some of them
are hardly applicable in mountain areas or
may be justified only locally. Some others
are currently used over large areas, including the alpine region. An integrated disease
management approach could be more efficient than single methods in controlling
these diseases.
Trenching
Digging isolation trenches around diseased
trees to prevent the secondary, vegetative
spread of root and butt rot fungi is one of the
most traditional control methods recommended against several pathogens including
H. annosum sensu lato and A. mellea sensu
lato (Korhonen et al., 1998b; Kliejunas et al.,
2005; Legrand et al., 2005; Eyles et al., 2008).
The trench should be at least 70–100 cm
deep. Instead of an open trench, it could be
more practical to bury a plastic sheet in a
vertical position into the soil (Korhonen
et al., 1998b; Legrand et al., 2005). As suggested by Eyles et al. (2008), the effectiveness of this method depends on the regular
maintenance of trenches, to prevent the reestablishment of root contacts, and on the
proper siting of the trenches to ensure all
infected trees are isolated. It is usually quite
difficult to determine whether or not trees
are infected; colonization of the root systems may go undetected for long periods of
time. Trenching is a very impractical control method and its use is advisable only
when pathogen inoculum is very localized.
It should be noted that in the case of
pathogens that are also able to spread aerially, trenching would offer limited protection. In the case of H. annosum, for instance,
observational data suggest that instead of
controlling the disease, trenching could
actually promote the spread of the fungus
by breaking and injuring roots (Korhonen
et al., 1998b).
Tree and stump removal
Secondary mycelia of most root and butt rot
fungi will survive and produce basidiomata
for a long period of time on colonized wood.
Thus, infected trees should be removed
from the stand promptly in order to reduce
the airborne inoculum of fungi spreading
through spores. As most of these fungi can
attack timber, asymptomatic felled trees
should also be removed. In general, timber
is unselective or less selective than standing
trees to infection by wood decay fungi
(Rayner and Boddy, 1986). As an example, S.
sanguinolentum, which commonly attacks
Norway spruce trees, is reported to colonize
felled wood of spruce but also of pine and
silver fir, in which it causes a red streaking
(Butin, 1995). Thus, attention should be
given not only to preferential hosts, but also
to other tree species that can become saprophytically colonized.
Removing stumps and roots from the
soil has been recommended for controlling
both H. annosum sensu lato and A. mellea
sensu lato (Korhonen et al., 1998b; Legrand
et al., 2005). Benefits of this method have
been reviewed recently by Vasaitis et al.
(2008). It should be noted that de-stumping
can show effectiveness not only against diseases spreading from tree to tree through
root grafts and contacts, but rather it may be
effective against a wide range of wood decay
Controlling Root and Butt Rot Diseases
agents. In fact, for instance, basidiomata of
P. schweinitzii growing on stumps are
reported as sources of soil infestation for
long periods of time (Barrett, 1985). Stump
removal is an expensive, time-consuming
control method (Korhonen et al., 1998b;
Legrand et al., 2005) that requires the use of
machines (Omdal et al., 2001). This method
can be adopted optionally in certain artificial
plantations after clear-felling. It is rarely used
in mountain forests or after selective cuttings.
Furthermore, de-stumping contrasts with current trends in forest management since it may
have negative effects on biodiversity.
Promoting tolerant species
If a forest is heavily infested by a root or butt
rot agent with restricted or defined host
range, control may be achieved by increasing the proportion of trees more resistant to
the pathogen. In principle, a rotation of a
resistant tree species can clean the site of
the pathogen inoculum (Korhonen et al.,
1998b). The concept of forest rotation has
been widely advocated for the management
of root rots of forest trees, with contrasting
results (Korhonen et al., 1998b; Lygis et al.,
2004). The pathogen inoculum usually persists in stumps and roots for decades after felling, and this was reported not only for H.
annosum or A. mellea species complexes
(Korhonen and Stenlid, 1998; Guillaumin and
Legrand, 2005) but also for other root and butt
rot fungus, i.e. P. schweinitzii (Barrett, 1985).
Changes in tree species composition,
turning a susceptible forest into a more tolerant one, can be achieved in a relatively
short period of time through clear-felling
only, followed by artificial plantation. In naturally regenerated, uneven-aged or irregular
alpine forests, shifts in tree species composition are more difficult to obtain and
they should be driven by appropriate silvicultural practices. Obviously, in such conditions, for a reduction of the pathogen
inoculum to occur, several decades, or even
centuries, may be necessary. As for most
plant diseases, it could be advisable to avoid
the complete removal of the susceptible
species, since the resulting selection pressure
353
on the pathogen might result in the infection of plants normally considered as nonhost (Garbelotto, 2004).
This method of control might show
some effectiveness in controlling Heterobasidion root and butt rots. In a recent unpublished study conducted in the western
Italian Alps, and based on the analysis of
about 2300 recently felled trees, it was
found that disease incidence was significantly higher on Norway spruce (43% of
average incidence) than on other native tree
species (Table 26.3). Silver fir was also
rather susceptible, while larch and, especially, Scots pine trees were more tolerant.
Moreover, with the exception of H. annosum sensu stricto, a strict host preference of
H. annosum species has been reported in
the Alps (Gonthier et al., 2001). Thus, based
on these data and on other observational
data (Table 26.3), it is likely, for instance,
that the establishment of deciduous tree
species would have beneficial effects in
most Heterobasidion-infested forests, but
not necessarily in H. annosum sensu stricto
infected stands (Table 26.3). Also, the control of H. abietinum in heavily infected silver fir forests could be achieved either by
promoting Scots pine trees or Norway
spruce trees. The disease in severely damaged Norway spruce stands can be lowered
by favouring the more resistant larch.
A. mellea sensu lato species display a
lower degree of host preference with respect
to H. annosum sensu lato species. Nevertheless, variation of tree species composition could have some beneficial effects for
this pathosystem also. For instance, Swiss
stone pine trees have been reported as very
susceptible to A. ostoyae in subalpine forest
(Anselmi and Lanata, 1989). Cuttings promoting the regeneration and establishment
of the most tolerant larch could be effective
in the management of Armillaria root rots in
these high elevation forests.
Timing of thinning and cutting
The timing of logging and harvesting may
have a strong impact on the incidence of
354
P. Gonthier
Table 26.3. Susceptibility of native alpine forest trees to H. annosum species based on the author’s
experience and on previously published (Gonthier et al., 2002, 2003) and unpublished data. The list does
not include susceptibility and symptoms of seedlings.
Tree species
Abies alba
Picea abies
Larix decidua
Pinus cembra
P. sylvestris
P. uncinata
Broadleaves
Average
Heterobasidion
incidence1
17%
43%
12%
15%
4%
?
?
H. parviporum
H. abietinum
H
H
H
H
++++
++
++
H. annosum sensu
stricto
+++
H
H
H
M
M
–
++
+
++
++
++?2
+
Note: 1Based on the analysis of about 2300 recently felled trees from 22 forest stands in the western Italian Alps;
2symptoms and tentative susceptibility according to Bendel et al., 2006. Symbols: +, recorded; ++, occasionally
diseased; +++, susceptible; ++++, very susceptible; M, root rot and mortality; H, heart rot; –, saprophyte.
airborne infectious diseases. As a rule, operations should be done preferably when environmental conditions are unfavourable to
the pathogen. For H. annosum sensu lato,
winter thinning and logging operations have
been used in Fennoscandia to take advantage of the low inoculum pressure during the
cold season (Brandtberg et al., 1996; Piri and
Korhonen, 2008). In that area, infections follow a bell-shaped curve with very low spore
deposition rates in winter; an average of only
2% of Norway spruce stumps was infected
following thinning in November–February
compared with 34% in June–July (Brandtberg et al., 1996).
Seasonal patterns of spore deposition
of Heterobasidion species have been studied in the alpine region recently with the
aid of woody traps (Gonthier et al., 2005).
Here, the airborne inoculum of this pathogen, although present starting in February at
most sites, is higher in August–October,
reaching a peak in September. A relative peak,
lower than the late summer one, appears in
late spring (Fig. 26.1). Thus, despite the
development of perennial basidiomata, in
the Alps the inoculum production of Heterobasidion spp. is concentrated largely in a
period of 2–3 months. Spore inoculum in
winter, spring and early summer is generally low (Gonthier et al., 2005). Although
the hazard of stump infection is not always
described accurately by spore loads on
woody traps (Driver and Ginns, 1969), recent
unpublished data confirm the above seasonal
patterns of spore deposition and indicates
that the highest risk of stump infection
occurs in autumn (Gonthier and Nicolotti,
unpublished).
Winter operations may not be feasible
at all sites in the Alps. However, Heterobasidion primary infections would be controlled successfully by planning logging
and thinning in most of spring and in early
summer. Although the above timing may be
somewhat impractical, its advantage in Norway spruce includes limiting infection
through wounds not only by H. annosum
sensu lato but also by S. sanguinolentum,
whose annual basidiomata are produced in
autumn (Solheim, 2006).
Biological and Chemical Control
Several biological and chemical methods
have been tested for the control of root and
butt rot fungi, especially of H. annosum sensu
lato and A. mellea sensu lato (reviewed in
Holdenrieder and Greig, 1998; Pratt et al.,
1998; Guillaumin et al., 2005a,b). Most
Controlling Root and Butt Rot Diseases
355
100
90
Infected traps (%)
80
70
60
50
40
30
20
10
0
JAN
FEB
MAR
APR
MAY
JUN
JUL
Months
AUG
SEP
OCT
NOV
DEC
Fig. 26.1. Average percentage of woody traps infected monthly by Heterobasidion spores in four forests
of the western Alps from 1998 to 2000. Re-elaborated from data published by Gonthier et al. (2005). Bars
show standard errors.
experiments were conducted in vitro. Currently, only a very few control approaches
are recommended in practical forestry and
they are all devoted to H. annosum sensu lato.
Stump treatment with appropriate biological
or chemical products immediately after felling may prevent H. annosum primary infections, therefore reducing timber losses.
A number of fungi have been tested on
stumps as competitors or antagonists throughout North America and Europe (Holdenrieder and Greig, 1998), including in the alpine
forests (Nicolotti et al., 1999). Only Phlebiopsis gigantea (Fr.) Jül is used currently,
with good results over large areas (Holdenrieder and Greig, 1998; Thor, 2003; Berglund
and Rönnberg, 2004; Thor and Stenlid, 2005).
Three distinct products based on this saprotrophic fungus have been developed: PG
Suspension® in the UK, PG IBL® in Poland
and Rotstop® in Fennoscandia. Rotstop®
showed a very good effectiveness in alpine
Norway spruce stands heavily infected
by Heterobasidion (Nicolotti et al., 1999;
Nicolotti and Gonthier, 2005). In a recent
comparative study performed in Austrian
alpine protection forests, strains of Phlebiopsis gigantea from Poland resulted in a
higher colonization frequency of spruce
stumps with respect to the Rotstop® strain
(Cech et al., 2008).
Several chemicals proved to be effective as stump protectants against Heterobasidion airborne infections (Pratt et al., 1998),
the most known of which are urea in Europe
and borax in North America. Both compounds, as well as other chemicals, were
tested in the western Alps on spruce stumps
and they showed very good results, comparable to those obtained with the Rotstop®
treatment (Nicolotti et al., 1999; Nicolotti
and Gonthier, 2005). The effectiveness of
urea was dependent on the concentration of
the water solution: the best results were
obtained with a 30% concentration. The
rise of pH of stump surfaces, which occurs
commonly during hydrolysis after treatment, rather than a toxicity of urea or ureaderivate compounds per se (e.g. ammonia
and ammonium ions), is responsible for the
inhibition of Heterobasidion germination
and growth (Johansson et al., 2002). Such a
high urea concentration allows high pH
values on stumps to be maintained for at
least the length of time these remain susceptible to infection, i.e. approximately 1 month.
356
P. Gonthier
Because the costs of registration are very
high, it is unlikely that any biological or
chemical product will be registered in the
near future for stump treatments in the
alpine area. However, urea and borax are
currently classified as fertilizers and their
use is mostly unregulated for forestry purposes (Nicolotti and Gonthier, 2005). Urea
could be preferred for its long history in
stump treatment in Europe (Nicolotti and
Gonthier, 2005; Oliva et al., 2008) and for
its moderate effects on non-target organisms
inhabiting stumps (Table 26.4). Furthermore, urea is effective on stumps of several
native alpine tree species (Gonthier and
Nicolotti, unpublished).
Integrated Disease Management
and Forest Protection:
A Concluding Example
It is generally agreed that systems combining cultural, biological, chemical or other
methodologies to reduce parasites are more
effective and even cheaper than single control methods. In the field of forest trees,
integrated pest management (IPM) systems
have been developed especially for the protection of forests against insects or nurseries
against diseases (Volney and Mallett, 1998;
South and Enebak, 2006). Appropriate IPM
systems may be developed only with a good
understanding of the pathogen biology and
disease epidemiology. Except for a few
pathosystems (i.e. H. annosum sensu lato,
A. mellea sensu lato), our current understanding of the epidemiology of root and
butt rot diseases is still limited to allow the
development of efficient IPM systems. However, the biology and epidemiology of H.
annosum sensu lato are well known and
some weak points exist in its life cycle (i.e.
stage of infection by spores).
An integrated management system was
designed to control H. annosum root and
butt rots in the Aosta Valley, western Italian
Alps. The system is based on stump treatment
Table 26.4. A summary of the effectiveness and impact on non-target fungi of biological and chemical
treatments against Heterobasidion airborne infections on Norway spruce stumps in the western Alps.
Antagonist/competitor or
active ingredient
Biological
Hypholoma fasciculare
Phanerochaete velutina
Phlebiopsis gigantea
Vuilleminia comedens
Verticillium bulbillosum
V. bulbillosum
Trichoderma harzianum
Chemical
Copper oxychloride
Propiconazole
Sodium tetraborate
decahydrate
Urea
Urea
Urea
Effectiveness
against
Heterobasidion1
Impact on
non-target fungi2
Wheat mash
Wheat mash
Rotstop®
Wheat mash
Culture filtrate
Conidial and mycelial
suspension
Conidial and mycelial
suspension
Low
High
High
Low
Medium
Medium
Low (after 2 years)
Low (after 2 years)
High
Low (after 2 years)
Very low
Very low
Low
Very high
Azuram®
TILT (25% emulsion)
Borax powder
High
High
High
Low (after 2 years)
Low (after 2 years)
Very high
Water solution 10% conc.
Water solution 20% conc.
Water solution 30% conc.
Low
High
High
Very low
Low (after 2 years)
Low (after 2 years)
Application method or
commercial product
Note: 1Categories (low, medium, high) were designed based on previously reported results (Nicolotti et al., 1999;
Nicolotti and Gonthier, 2005); 2categories (very low, low, high, very high) were designed based on previously reported
results (Varese et al., 1999; 2003a,b).
Controlling Root and Butt Rot Diseases
with urea at 30% concentration, combined
with an appropriate timing of thinning and
logging operations, and with practices
aimed at promoting tolerant species (Fig.
26.2). In the Aosta Valley, forest management activities are planned yearly by the
Regional Forest Administration, who
decides the stands that need to be thinned
each year. Every forest harvesting team is in
charge of thinning a variable number of
stands (3–8). While planning the timing of
thinnings, priority is given to the most susceptible (see Table 26.3) and heavily
infected stands, which are thinned preferably in spring and early summer, when the
risk of stump infection is still limited. The
average minimum air temperature of a
4-week period has been identified as a suitable predictor for modelling Heterobasidion
primary infections in the Alps (Gonthier
et al., 2005) and may be used for an accurate
estimation of the seasonal risk of stump
infection for each forest stand. Remaining,
less susceptible and uninfected stands are
thinned in summer or autumn. Stump treatment is necessary during summer and
autumn thinnings and is strongly recommended whenever dealing with uninfected
or susceptible stands, regardless of their
location and distance from an infection
source. In fact, despite a general limited
Winter
?
Highly susceptible
stands
Spring
potential dispersal range of Heterobasidion
spores (Gonthier et al., 2001), with spore
densities undergoing huge dilution after the
first metres (Stenlid, 1994), the migration of
even a few spores may be significant for
areas still not colonized by the pathogen
(Garbelotto, 2004).
Winter operations may be possible
locally, in low elevation stands. Sanitation
fellings are advisable in Scots pine forests to
reduce bark beetle attacks. Depending on
the forest function and on the economic
injury level, practices such as de-stumping
and, especially, the transformation of heavily infected susceptible forests into more
tolerant ones can be arranged locally and
they may be suited. For instance, increasing
the larch component in subalpine spruce
forests would have positive effects not only
in reducing Heterobasidion incidence, but
also in improving general forest stability
(Motta and Haudemand, 2000). At lower
elevations, the regeneration of diseased
spruce forests with silver fir or Scots pine
could be advisable since H. parviporum
very seldom attacks mature firs or pines
(Korhonen et al., 1998b).
Obviously, any integrated disease management system to fight root and butt rots of
forest trees should fit and meet the requirements of the general forest management
Summer
Promote
tolerant tree species
Autumn
Stump treatment
ST
Non-susceptible
stands
357
Heavily infected
stands
Uninfected stands
Sanitation cuttings (DT, WC)
de-stumping (WC)
Fig. 26.2. Diagram of the integrated disease management system developed to fight Heterobasidion
root and butt rots in the Aosta Valley, western Italian Alps. Arrows indicate the appropriate timing of
thinning. Stump treatment is performed with urea at 30% concentration. Symbols: ?, where possible;
ST, stump treatment; DT, dead trees; WC, where convenient.
358
P. Gonthier
system of the area (Tainter and Baker, 1996).
Within an integrated forest protection approach, the integrated disease management
system here described could combine other
pest management systems, for instance those
designed for the control of Ips typographus
L. or other bark beetles threatening alpine
forests.
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27
Some Important Fungal Diseases and
Their Impact on Wheat Production
Aakash Goyal and Rajib Prasad
Agriculture and Agri-Food Canada, Lethbridge Research Center,
Lethbridge, Canada
Abstract
Wheat, an important cereal crop, is cultivated worldwide and is second highest in production, just
after maize. Due to the increasing world population, there is need for a 40% increase in wheat production to meet global food requirements. Wheat production is diminished mainly by biotic and abiotic
stresses all over the world. Of these, pathological diseases are the most important limiting factor of
wheat production as different pathogens infect wheat plants, causing severe losses in yield and quality. Wheat can be infected by biotrophic fungi, necrophytic species and nematodes, as well as viruses
and bacteria. Among these, different fungal diseases are the most prominent and pose a great challenge
to wheat production. Development of resistant varieties is the only solution to overcome this problem
and to attain the required wheat production. The development of resistant varieties has benefited
immensely from the use of molecular markers, genetic maps, physical maps, QTL analysis and markerassisted selection (MAS). However, we have to develop multidisease-resistant varieties to fulfil the
demand for wheat globally. This review highlights some major fungal diseases of wheat in different
parts of the world and the associated problems.
Introduction
Common wheat (Triticum aestivum L. em.
Thell) is an important staple food crop and
ranks first among the three major crops
(wheat, maize and rice), which together
constitute about half of the total world food
production. Wheat feeds about 40% of the
world population and provides 20% of the
total food calories and protein in human
nutrition (Varshney et al., 2006). It is not
only used for bread making but is also used
for making biscuits, cakes, breakfast cereals,
pasta and fermented products like beer,
alcohol, vodka, etc. It is also becoming popular as a forage crop. Wheat straw has been
362
used as a fodder material and for ethanol
production for the past few years. Unlike
rice and maize, which prefer tropical environments, wheat is best adapted to temperate regions, occupying 17% (one-sixth) of
the total crop acreage worldwide (Gupta
et al., 2008). According to the FAO (2007),
wheat occupies 20% of the cultivated crop
area (in 2007, 213m ha versus 150m for rice
and 143m for maize) and its annual production is 619 Mt of grain. Over the past 20
years, there has only been a small increase
in the area of land on which wheat is cultivated worldwide, but the tonnage of wheat
grain produced on this land has tripled as a
result of improved farming practices and
CAB International 2010. Management of Fungal Plant Pathogens
(eds A. Arya and A.E. Perelló)
Some Important Fungal Diseases and Wheat Production
the development of better wheat varieties
(Marshall et al., 2001). A significant increase
in wheat production has been observed in
the past four decades; however, a slowing
down has been witnessed during the past
few years (Gupta et al., 2008). Due to a consistent increase in world population, there
is need for a 40% increase in wheat production to meet this requirement. Despite the
enormous progress that has taken place
around the world, there is less hope in
achieving this goal. Resistance to both biotic
and abiotic stresses will be critical for reaching this target. Abiotic stresses include
drought, untimely or excess heat, untimely
or excess rain, water logging of soils, wind,
extreme cold, frost, acid soils and salinity,
nutrient imbalances and/or shortages, as
well as micronutrient deficiencies.
The impact of biotic stress on wheat
production and quality is highly devastating. Diseases in wheat, most caused by
fungal pathogens and a few by viruses and
bacteria, are important production constraints in almost all wheat-growing environments (Rajaram and van Ginkel, 1996).
Wiese (1987) identified over 40 fungal, 32
viral and 81 bacterial diseases that attack
wheat plants at different growth stages.
Although it is difficult to obtain accurate
estimates of crop losses to different fungal
diseases, the British Agrochemicals Association (1993) suggests that, under farm conditions where crop rotations, good husbandry
and the application of pesticides are practised, losses to diseases can still be around
13%, while under conditions where crop
protection measures are not taken, losses
can be as high as 50%. It is the goal of wheat
breeders to introduce genetic resistance into
their varieties to minimize chemical protection measures and losses due to diseases.
Under different environments, breeders face
problems of different spectra of locally prevalent diseases caused by specific biotypes,
serotypes and strains. For many diseases,
genes for resistance segregating in a simple
‘Mendelian’ fashion have been identified;
while for other diseases, resistance genes
still remain to be detected, due to either a
complex mode of inheritance or imprecise
disease-screening procedures (Gowda et al.,
363
2006). Recently, available molecular markers
and functional genomics tools have helped
the breeder to manipulate the wheat genome
to develop disease-resistant cultivars and
achieve the target of wheat production.
Fungal Diseases of Wheat
Most of the important diseases of wheat are
caused by fungal pathogens, while only a
few are caused by viruses and bacteria
(McIntosh et al., 1995; Rajaram and van
Ginkel, 1996). Infection of fungal diseases
in wheat depends on the availability of free
water on the host plant surface, susceptibility of the host, the density of inoculum, temperature and other environmental factors.
Moreover, host–parasite interaction plays a
significant role in the development of disease and subsequent symptoms on the wheat
plant. In this chapter, some of the commonly reported fungal diseases of wheat are
described. Table 27.1 lists major fungal diseases of wheat reported by different pathologists around the world.
Fusarium head blight (FHB)
Several species of Fusarium can cause Fusarium head blight (FHB), also known as scab of
cereal crops. Among these, F. graminearum,
found mainly in the USA, Canada, China
and the EU, is accountable for severe losses
in yield and quality of wheat production
(Parry et al., 1995). An epidemic of FHB in
the USA and Canada in 1993 was a result of
changes in crop management practices (minimum or reduced tillage), changes in rainfall
patterns and a low resistance in the cultivars against FHB (Dill-Macky and Jones,
1997). In the case of wheat, Fusarium spp.
attacks different plant organs but mainly
targets the ear, which leads to great loss in
seed quality. On the ear, Fusarium enters
through the stomata to the palea and lemma
and destroys these tissues completely. The
first symptom of FHB is a tan or brown discoloration at the base of a floret within the
spikelets of the head. The infection may be
364
A. Goyal and R. Prasad
Table 27.1. The major fungal diseases of wheat reported across the world.
Name of the
disease
Pathogenic fungal
species
Tolerant varieties/
genotypes
References
Black point/kernel
smudge
Common bunt
Common root rot
Ergot
Fusarium head
blight (scab)
Leaf rust
Alternaria alternata
Sunco, Cascades
Lehmensiek et al., 2004
Tilletia caries, T. foetida
Cochliobolus sativus
Claviceps purpurea
Fusarium graminearum
AC Domain
ND 652
Carleton, Kenya farmer
Bizel, Sumai 3
Fofana et al., 2008
Mergoum et al., 2005
Platford and Bernier, 1970
Bourdoncle and Ohm, 2003
Puccinia recondita
(P. triticina)
Tangmai 4, ND 652
Loose smut
Powdery mildew
Speckled leaf
blotch
Glume blotch
Spot blotch
Stem rust
Ustilago tritici
Erysiphe graminis
Septoria avenae
f. sp. tritici
Stagonospora nodurum
Cochliobolus sativus
P. graminis f. sp. tritici
DT676
Tangmai 4
Arina and Riband;
Courtot and Tonic
Red Chief
Ning 8201, K8027
ND 652, Tangmai 4
Li et al., 2004;
Mergoum et al., 2005;
Kolmer et al., 2007
Knox et al., 2008
Li et al., 2004
Chartrain et al., 2009
Stripe rust
Take-all
P. striiformis
Gaeumannomyces
graminis var. tritici
Tangmai 4
Xinong 1376,
Xinong 918, R859
limited to one spikelet, but if the fungus
invades the rachis, the entire head may
develop symptoms of the disease. Discoloration of the head starts due to the production
of mycotoxins [zearalenones and deoxynivalenol (DON)] by the Fusarium. The mycotoxins affect seed quality adversely, producing
toxic dust and thus making the seeds unsuitable for human and livestock consumption
(Eudes and Laroche, 2003). The mycotoxin
DON, even in low doses of 1–3 ppm, can
cause reduced feed intake and less weight
gain in animals, while a high dose up to
10 ppm can cause vomiting and refusal to
feed. DON is also very harmful to humans;
therefore, different countries have established laws to protect consumers. For example, the EU Member States allow a maximum
of 1.25 ppm DON in unprocessed bread,
0.5 ppm in bread and bakery products and
only below 0.2 ppm in baby foods (Buerstmayr et al., 2009). The USA Food and Drug
Administration recommend only 1 ppm
DON in finished wheat products, while
Health Canada have established guidelines
Laubscger et al., 2008
Sharma et al., 2007
Li et al., 2004;
Mergoum et al., 2005
Li et al., 2004
Xiaoning et al., 2004
of 2 ppm and 1 ppm DON in soft wheat in
non-stable and baby foods, respectively.
Control of this disease has been difficult, because of the complex nature of the
host/pathogen interaction. Cultural practices, such as rotation with non-host crops
and management of crop residues, in combination reduce primary infection. A mixed
fungicide composed of carbendazim and triadimefon was reported to have a significant
synergistic action (Wang, 1997). Under high
disease pressure, Bravo or Folicur were
reported to reduce levels of FHB, though
these are not cost-effective under low disease pressure (Agrios, 1997). Host resistance
is a promising and effective management
solution, but resistance has not been easy to
achieve in the adapted cultivars.
Wheat rust
Wheat rust pathogens belong to genus Puccinia, family Pucciniaceae, order Uredinales and class Basidiomycetes. Rust disease
Some Important Fungal Diseases and Wheat Production
is capable of causing considerable economic
loss throughout the world (FAO, 2008). Rust
in cereals, found back in the late 17th century, was caused by a fungal parasite which
was named later as Persoon’s P. graminis
(Chester, 1946). In the beginning of the 20th
century, different fungal species were identified for different rusts with contrasting host
ranges. In wheat, rust diseases are so important that in 2007, the CSIRO, Australia, published a special issue on wheat rust in the
Australian Journal of Agricultural Research.
Stem rust
Stem or black rust of wheat is a major disease
problem, caused by the fungus, P. graminis
Pers. f. sp. tritici. It has been a major disease
on wheat since the rise of agriculture and the
Romans even prayed to a stem rust god,
‘Robigus’. The Italians, Fontana and Tozzetti,
independently provided the first report on
stem rust in wheat in 1767. In the early to
mid 1950s, stem rust epidemics caused
approximately 50% yield losses of wheat in
North America (Leonard, 2001). During the
1950s, Norman Borlaug and other scientists
started developing high-yielding wheat varieties that were resistant to stem rust and
other diseases in North America and throughout the world. The rust-resistant, highyielding wheat variety banished chronic
hunger in much of the world, ended stem
rust outbreaks and won Borlaug the Nobel
peace prize in 1970 (Singh et al., 2006). In
most areas of the world, the life cycle of P.
graminis consists of continual uredinial
generations. The disease spreads either via
airborne spores or occasionally locally from
wild susceptible barberry (Berberis sp.)
plants (Eversmeyer, 2000).
Ug99, so called as it was first seen in
Uganda in 1999, is a new devastating race of
‘stem rust’ which has already travelled from
Africa to Iran and can proceed to India, Pakistan and Bangladesh (Pretorius et al., 2000).
It is particularly dismaying because of its
ability to infect crops in just a few hours
and its vast cloud of invisible spores can be
carried by the wind for hundreds of miles
(Singh et al., 2006).
365
Cultural control provides at least partial control of wheat rust epidemics. Planting early maturing varieties is an efficient
way to avoid losses due to stem rust infection. Propiconazole (Tilt) and triadimefon
(Bayleton) are found to be effective against
stem rust (Agrios, 1997), though these chemicals are cost-prohibitive. To save the world
from the wheat epidemic, CIMMYT and
ICARDA started the Global Rust Initiative
(GRI) to coordinate efforts to track and study
Ug99 and develop resistant varieties of wheat
(Stokstad, 2007). Later in 2008, it was taken
over by the Borlaug Global Rust Initiative
(BGRI), chaired by Dr N.E. Borlaug, who said
he was optimistic that the fungus would be
beaten again (Stokstad, 2007). Efforts were
also taken to understand the rust’s epidemiology and evolution, which led to the
barberry eradication programme in North
America and Europe (Singh et al., 2006).
Leaf rust
Wheat leaf rust, also known as brown rust,
is caused by the rust fungus, P. triticina Rob.
Ex Desm. f. sp. tritici Eriks (syn. P. recondita). De Candole (1815) reported for the first
time that leaf rust was caused by fungus and
named it Uredo rubigovera. Later in the
19th century, the name was changed to P.
recondita (Cummins and Caldwell, 1956).
However, the present name, P. triticina, was
suggested by Savile (1984) and Anikster
et al. (1997). Up to 2007, more than 50 races
of leaf rust were detected all over the world
(Kolmer et al., 2007; Mebrate et al., 2008).
Leaf rust is the most prevalent of all the
wheat rust diseases, occurring in nearly all
areas where wheat is grown. Depending on
the severity and duration of infection, losses
in wheat can vary by up to 50% (Nagarajan
and Joshi, 1975; McIntosh et al., 1995). The
disease has caused serious epidemics in
North America, Mexico, South America and
some other countries. This fungus can infect
wheat plants with a 3 h dew period at temperatures near 20°C. However, more infections occur with longer dew periods. The
fungus initially starts covering leaves with
366
A. Goyal and R. Prasad
orange pustules of urediniospores (uredinia).
The urediniospores are reddish-brown, elliptical to egg-shaped, echinulate structures. In
the later stage, the postules eventually darken
due to the formation of black teliospores
(Roberson and Luttrell, 1987). Infections can
result in a 1–20% yield loss since infected
leaves die earlier and all the nutrients are
directed to the growing fungi. Infection can
also cause grains to shrivel. The loss in yield
depends on several factors that include time
of initial infection, crop development stages,
relative resistance or susceptibility of the
wheat cultivars. Higher yield losses result
when the initial infection occurs early in
the growing season before tillering. Infection occurring after heading when grain filling is in progress will cause lesser crop loss
(Agrios, 1997).
Chemical control with trizole fungicides has been reported as useful in controlling infections up to ear emergence, but is
difficult to justify economically in attacks
after this stage. Varietal control is again the
best control for leaf rust. Resistant varieties
possess one or more special leaf rust resistance genes called Lr genes. Currently, there
are more than 58 different Lr genes available in wheat (Bansal et al., 2008; Chhuneja
et al., 2008; McIntosh et al., 2008), but most
varieties have only a few Lr genes. So, there
is a need to develop multi Lr gene-carrying
varieties to defeat leaf rust disease.
Yellow rust or stripe rust
Another rust of wheat, stripe or yellow rust
which is caused by P. striiformis f. sp. tritici,
can be as damaging as other rusts. Due to a
requirement for a very low optimum temperature for its development, stripe rust is
not found in many areas of the world. However, a total area of 9.4m ha (> 35%) under
wheat cultivation is affected by stripe rust
(Singh et al., 2004). On the world level,
stripe rust is found predominantly in northern Europe, the Middle East, East Africa,
China, India and the continents of South
America, Australia and New Zealand (Saari
and Prescott, 1985). In the USA, it was first
reported in 1915 (Carleton, 1915) and serious outbreaks were reported in the western
states in the 1960s (Line, 2002; Boyd, 2005).
For this disease, generally no cultural
control measures are applicable, but in the
USA, where the disease occurs commonly,
the removal of the alternate host is an established method of cultural control. Identification and use of the resistant gene in
resistant varities is the only way to reduce
the impact of the disease on wheat production. Many yellow rust resistance genes have
been identified in wheat by different wheat
workers and to date, 41 of these (Yr1 to Yr41)
have been designated (McIntosh et al., 2008).
Most of the identified yellow rust resistance
genes have proven to be race-specific, with
resistance being effective only against isolates of P. striiformis f. sp. tritici carrying
the corresponding avirulence gene. Different
wild wheat varieties were also used to transfer the resistance gene to hexaploid wheat
for stripe rust resistance (Kuraparthy et al.,
2007a,b; Singh et al., 2007; Chhuneja et al.,
2008). More recently, a highly resistant gene
with broad spectrum on strip rust races,
namely Yr36, from wild emmer wheat was
used for positional cloning (Fu et al., 2009).
Karnal bunt
Karnal bunt (partial bunt) of wheat has
become a disease of serious concern in some
parts of the world as it causes direct yield
losses and also has significance as an export
problem because many believe the pathogen to be a quarantine pest. Consequently,
stringent quarantine measures have been
adopted in several countries, which may
affect not only the wheat grain trade but also
germplasm exchange (Royer and Rytter,
1988). Karnal bunt caused by the smut fungus Tilletia indica Mitra Neovossia indica
(Mitra, 1931), a Basidiomycetes fungus, is a
serious floral-infecting disease of wheat in
the major wheat-growing areas of India
(Gill, 1990) and some other wheat-growing
countries of the world (Nath et al., 1981).
The pathogen is known to infect bread
wheat, durum wheat and triticale (Agarwal
Some Important Fungal Diseases and Wheat Production
et al., 1977). The disease was first reported
in 1931 in experimental wheat crop at the
Botanical Station at Karnal, India (Mitra,
1931), and was for many years known only
in the plains of India and Pakistan (Ahmad
and Attaudin, 1991). Currently, it occurs in
Afghanistan, India, Iran, Iraq, Mexico, Nepal
and Pakistan and in limited areas of the USA
(Durán, 1972; Munjal, 1975; Singh et al.,
1989; Ykema et al., 1996). Recognition of
fungal structures (teliospores) on grain samples from Lebanon and Syria suggest that
the disease is established in these countries
as well (Locke and Watson, 1955).
Karnal bunt requires free water in the
soil for teliospores, the overwintering life
stage of Karnal bunt, to germinate. Teliospores are brown to dark brown, spherical
or subspherical, or oval, 22–42 × 25–40 µm
in diameter, occasionally having an apiculus (Roberson and Luttrell, 1987), papilla
(Mitra, 1931) or a vestige of attached mycelium (Durán and Fischer, 1961). The disease
cycle (Fig. 27.1) starts with the introduction
of teliospores on to a field. Contaminated
seeds are considered to be the major source
of teliospores, while other sources include
wind, animals, contaminated equipment or
contaminated vehicles. Teliospores may
remain dormant but viable for several years
(Ottman, 2002). Although planting infected
seed is the primary means of getting the
spores into the soil, this may or may not
produce infected plants directly in the first
year. The greater threat of disease occurs the
following year as the soil is turned over,
bringing these teliospores back to the surface. At the flowering stage of host plants,
the teliospores produce sporidia that infect
the plant florets and fungal hyphae enter the
ovary (Aujla et al., 1977; Singh and Prasad,
1978; Khetarpal et al., 1980; Krishna and
Infected
grains
Primary
infection
(partial systematic spread)
Combining
and threshing
Germination
of soilborne
teliospores
Subsequent
spread to
late tillers
Multiplication
on wheat and
other plant leaves
367
Germination of primary
spordia on whorl
Allantoid
secondary sporidia
Fig. 27.1. The life cycle of Karnal bunt caused by Tilletia indica Mitra.
Filliform
secondary
sporidia
368
A. Goyal and R. Prasad
Singh, 1982). Subsequent disease development in the embryo end of the kernel results
in the formation of new teliospores, which
are deposited back in the soil at harvest,
adding further to soil inoculum. Cool,
cloudy and very humid conditions or rainfall between awn emergence and the end of
flowering are required for sporidia production, infection and for the disease to flourish
(Dhaliwal et al., 1983; Goates, 1988). The
incidence of Karnal bunt is usually very low
and rarely seen if the environmental requirements are not met. Karnal bunt affects the
heads of wheat plants. The disease is not
easily detected in the field because few florets are typically infected and the area of the
kernel affected might be small and facing
inwards. A mass of black teliospores is
found at the embryo end of the kernel and,
at higher levels of infection, along the crease
or in the entire kernel (Goel et al., 1977;
Dhaliwal et al., 1983). A fishy odour is emitted from infected seeds due to the presence
of trimethylamine (Mehdi et al., 1973).
Conventional approaches to control
this disease consist of the adoption of various cultural practices such as crop rotation
for longer periods, sowing of disease-free
seeds, adjustment of the nitrogen balance in
the soil and adjustment of the time of irrigation to minimize disease incidence (Mitra,
1937; Munjal, 1974; Goel et al., 1977; Singh
and Prasad, 1978; Aujla et al., 1981, Singh
and Singh, 1985; Gill et al., 1993). Control
through fungicides is not completely effective as the disease is seed- and soilborne
(Singh et al., 1985). However, application of
Tilt at heading and 1 week later can reduce
disease incidence by 90% when environmental conditions are conducive to disease
development (Ottman, 2002). Hence, the
most economical, eco-friendly and effective
approach to control the disease is the cultivation of resistant varieties. The main
sources of resistance against Karnal bunt
have been the Indian, Chinese and Brazilian
wheats (Fuentes-Davila and Rajaram, 1994).
A new range of genetic variability for resistance to Karnal bunt has been observed in
synthetic hexaploid wheat derived from T.
turgidum × T. tauschii crosses (Villareal
et al., 1996).
Powdery mildew
Powdery mildew of wheat, a wind-dispersed
disease, is an important and most common
disease worldwide, particularly in humid
regions (Oerke et al., 1994). It is of special
interest in epidemiology because it results
in reduced kernel size and seed weight, and
ultimately lower yield. The fungal pathogen,
Blumeria graminis f. sp. tritici (an Ascomycete), causing powdery mildew on wheat,
is a biotrophic obligate parasite (Cooke
et al., 2006), which is highly sensitive to the
environment and its presence can vary from
season to season (Jenkyn and Bainbridge,
1978; Jorgensen, 1988; Wolfe and McDermott, 1994). The powdery mildew fungus is
made up of different races and forms that
are highly specialized. Wheat cultivars
might be resistant to a certain race of the
mildew fungus, but susceptible to another
race. Some of the special features of powdery mildew, such as wide distribution,
rapid development within or on host tissue,
massive production of spores, the ability to
remain viable after long-distance dispersal
and a high capacity to become virulent on
previously resistant cultivars, make it a devastating disease of wheat (Boshoff et al.,
2002).
Powdery mildew oversummers on volunteer crops in the asexual stage, infects
the autumn-sown crop and, eventually,
overwinters on the volunteers to infect the
crops in spring (Zadoks, 1961). In mild
areas, volunteer wheat plants are abundant
because of the relatively frequent rainfall
in summer, while in dry regions, oversummering can depend on grass species
(Boshoff et al., 2002). For mildew, the asexual cycle is the production of haploid
conidia, while occasionally the ascospores,
which are the result of sexual cycle, can
initiate epidemics (Cooke et al., 2006). Mildew also differentiates a sexual stage,
which contributes to oversummering. In
early summer, B. graminis f. sp. tritici initiates the formation of generative mycelium
and cleistothecia starting on the lower
leaves. In the cleistothecia, 15–20 asci
develop, each containing eight haploid
Some Important Fungal Diseases and Wheat Production
ascospores which are dispersed by wind,
even under high humidity after rain (Gotz
et al., 1996). Ascospores can develop at any
time during the last half of the year; therefore, sexual reproduction is more important
for powdery mildew on wheat. Apart from
ascospores, conidia from the summer crop
can also infect volunteer plants; thus, a
mixture of ascospores and conidia forms
the inoculum for the winter crop. However,
the mildew population grown during
autumn on the winter crop can survive the
cold period in vegetative stage on overwintering green plants (Cooke et al., 2006).
Mildew is more severe in dense stands of
heavily fertilized wheat. Plants are most
susceptible during periods of rapid growth,
especially from stem elongation through
heading growth stages.
Powdery mildew on wheat is recognized by small, effuse patches (colonies) of
cottony mycelia on the upper and lower surfaces of the leaves. As these patches sporulate and age, they become a mass of dull tan
colour. Chlorotic (yellow) patches may later
surround the mildew colonies (Purdy, 1967;
Kingsland, 1982). Powdery mildew attacks
the leaves, but stems and heads are also
affected. The fungus grows primarily on the
surface of the host and feeds on the living
green cells of the plant. Damage occurs from
reduced photosynthetic ability when green
surfaces are shaded and the host is robbed
of moisture and food by fungal growth.
Yields may be reduced by 20% or more.
Spring wheat, other than soft white wheat,
are seldom affected at economic levels on
the prairies, while winter wheat is affected
to a greater degree. The disease will reduce
yields seriously if the flag and second leaves
are affected (Gotz et al., 1996; Boshoff et al.,
2002).
Incorporating wheat residues into the
soil, destroying volunteer wheat and crop
rotation can lessen the amount of overwintering inoculum in the field. Powdery mildew thrives where high rates of nitrogen
have been used. Therefore, use of a correct
and balanced fertilization programme with
proper levels of N, P and K is advised. It is
important to keep the top two leaves of the
plant as disease free as possible so that the
369
plant can use its full potential to fill the
grain. Fungicides can be applied based on
the level of disease in the field, the known
susceptibility of the variety and the selling
price of the grain (Agrios, 1997). Growing
mildew-resistant cultivars is the most economical way to control powdery mildew,
though wheat varieties vary in their resistance to powdery mildew and new races of
the fungus can attack previously resistant
varieties.
Conclusions
Wheat is a most important cereal crop and
is becoming more in demand due to the
significant increase in the world population. To protect the world from the upcoming threat of hunger, food and nutrition,
wheat production must be doubled in time.
Major concern about wheat quality and
production is related to biotic stress. Different methods, such as chemical control, cultural methods and eradication of alternate
hosts, are used to prevent the disease, but
the most important and effective one is the
development of resistant varieties. For
durability of resistance against fungal
diseases, breeders should focus on new
sources of race-specific resistance genes
from either adapted cultivars or wild varieties. However, extensive knowledge of the
pathogen population is a vital criterion in
assessing resistance and guidelines for
breeders to incorporate useful resistance
genes into the desired background. Recent
studies have proved the usefulness of different marker systems and association mapping of genes/QTLs controlling resistance
against different fungal diseases (Crossa
et al., 2007). Similarly, MAS was also
employed successfully to improve quality
and resistance against disease (see Dubcovsky, 2004; Anderson, 2007; Sorrells,
2007). In future, new molecular marker systems (e.g. ESTs, SNPs and DArTs) and functional genomics approaches (e.g. TILLING,
RNAi and epigenetics) can be used to facilitate the development of resistant varieties
in bread wheat.
370
A. Goyal and R. Prasad
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Index
Note: Page numbers in italic refer to tables and figures in the text
Abies spp., root and butt rot 347, 349, 353, 354
abiotic stresses
endophyte protection of plants 189–190
fenugreek 246–247
wheat 363
Acaulosporaceae 166
accelerated ageing germination (AAG) 332–333
acetaldehyde 8
acetic acid 43, 341
Achyranthes japonica 40
acibenzolar-s-methyle (ASM) 9
Acremonium terricola 284
Actinomycetes, antibiotics 123
Adgen Phytodiagnostic Septoria ELISA kit 299
aflatoxins 15, 29, 31, 39
production inhibited by plant extracts 39,
60, 61
recommended limits 34
agglutinins 127
Agrobacterium radiobacter 125, 125
agronomic characters, and disease resistance 73,
81–82
ajoene 20, 22
aldehydes 42
alfalfa 173, 207
alimentary toxic aleukia (ATA) 31
alkaloids 33, 150, 190
allicin 20, 22
Allium sativum extract 40
allyl-isothiocyanate (AITC) 43
Aloe vera gel 9
alpine European forests 345–346
ecological functions 345
management 350
root and butt rot
biological and chemical control
122–123, 354–356
diagnosis 350–351, 351
effects of forest management 350
general control strategies 351–354
infection biology 346–349, 347
integrated management 356–358
wood decay 348
tree species 345–346
Alternaria spp. 232
fruit crops 5
inhibition by plant extracts 40–41
leaf blight
castor 271–272
safflower 267–268
wheat 232–233, 241
leaf blight/black point 234–236
leaf spot
sesame 270
sunflower 265–266
Alternaria alternata 364
Alternaria carthami 271
Alternaria helianthi 266
Alternaria infectoria 234–236
Alternaria padwickii 40–41, 55
Alternaria sesame 270
Alternaria solani 173
Alternaria triticina 232–233
aluminium tolerance 189
375
376
Index
Amaranthus spp. 311–312
smut
incidence in cultivars 315, 316
wild species 313, 315–316
Amaranthus hybridus 315–316
Amaranthus retroflexus 315–316
AMF, see arbuscular mycorrhizal fungi
amino acids 177
anise 56, 60
antagonists 122
criteria for commercial production 111
endophytes 185–186
fruit storage pathogens 6–7, 110–111
intergration with other control measures
115–116
mechanisms of action 111–113
root and butt rot fungi 355
tan spot 283–284
antibiosis 9, 111, 123
KB-8A 270
Trichoderma 123, 127
antiseptics, stored produce 34
Aosta Valley forests 357, 357
Aphanoderma album 212
apoplast, protease activity 304
apples 6, 43, 174
hot air treatment 116
potassium iodide wraps 6
arbuscular mycorrhizal fungi (AMF) 124
effects of agricultural practices 163–165
importance in agriculture 162–163
interaction with fungal pathogens 122–123,
172–175
mechanisms of disease control 175–177
role in plant nutrition and growth 172
signalling pathway 175
soil propagule bank 164
effects of tillage 164–167
taxonomy 172
Argentina
fungicide use 292
wheat pathogens 231
Alternaria leaf blight 232–233
Alternaria leaf blight/black point
234–236
Ascochyta hordei leaf spot 236–237
Cephalosporium gramineum stripe
238–239
Cladosporium herbarum leaf spot
239–241
monitoring changes 241–242
Phoma soghina leaf spots 237–238
Pyricularia grisea spot blight 241
tan spot 276–284
wheat production 276, 291–292
arginine 177
Armillaria mellea sensu lato 347, 348, 349
control 133, 351–358
diagnosis 351, 351
aroeira extract 61
aromatic compounds, fruit and vegetables 7–8,
42–43
asarone 20, 22
Ascochyta hordei var. europaea 236–237
Ascochyta leaf spot 252
Aspergillus spp.
plant extract treatment 59–61
seed spoilage 330–331
Aspergillus flavus 29, 31, 32, 330
Aspergillus niger 265, 330
Aspergillus ruber 330–331
AspireTM 110, 114
Avr gene products 129
AVR-Pita avirulence gen family 98
Bacillus spp. 124–125, 252
bacteria
biocontrol of soil diseases 124–125, 125
endophytes 150
bacterial disease
endophyte plant protection 186–187
fenugreek 247, 248
bags, seed storage 336, 338, 340, 341
bajra, rusts 209
banana, mycorrhizae 176
bark beetle 357
bark extracts 40
barley, Fusarium head blight 79, 81–83, 87–88
barley yellow dwarf virus 187
basil extracts 40, 60–61
bavistin 332, 332, 333
bayletan 332
bean, rust fungi 209–210, 214, 216
beet, leaf endophytes 151–152, 153
benlate 332
benodanyl 214
benomyl 226
benomyl thiabendazole 34
benzaldehyde 42
benzanilide 214
benzimidazole 214
Beta vulgaris var. esculenta, see beet
biological control
airborne disease 124
commercial products and systems 125, 125
components 131–134
constraints in development 113–115
Trichoderma spp. 131, 133
definition 121–122
efficacy, consistency of 114
endophytes 150–151, 185–190
mechanisms 122–123
Phytophthora sojae 323
Index
postharvest diseases 17–18, 110
rusts 212
Septoria tritici blotch 303–305
soilborne diseases 124–125, 125
tan spot of wheat 159, 283–284
see also botanicals; essential oils;
Trichoderma spp.; yeasts
bioprotection, AM fungi 171, 172–175
BioSaveTM 7, 110
Bipolaris spp., seed pathogens 55–56
Bipolaris sacchari 223–224
Bipolaris sorokiniana 55
bitter leaf 40
‘black mars’ 29
black pepper 174, 176
black point, wheat (Alternaria infectoria) 234
black stem rust 203, 214
bleaching powder, seed treatment 333
blue mould decay 44, 116
Blumeria graminis f.sp. tritici 368–369
borax 355–356
bordeaux mixture 214
botanicals 7–9, 17–18
chemical structures 21–22
effective against toxin producing fungi 39
effects on seed fungi 55–61
efficacy in fungal control 19–20
essential oils 8–9, 18
seed treatments 38–39, 52–53
fruit crop disease control 7–9
potential advantages 45
potential risks 45–46
seed treatments 41
seedborne fungi
Alternaria 55
Aspergillus 59–61
Bipolaris 55–56
Colletotrichum 56
Curvularia 56–57
Fusarium 57–59
Macrophomina 59
Penicillium 59–60
Botryosphaeria spp. 133
Botrytis spp.
botanical control 42, 43–44
grey mould of fruit 4
grey rot of castor 272
Trichoderma control products 133
Botrytis cinerea 4, 42, 43–44
Botrytis ricini 272
brassicol 332
brinjal, seed treatments 334, 335, 335
Bromus spp., head smut 139–140, 140, 141
brown rot 4
brown rust, wheat 364, 365–366
Bt genes 342
Burkholderia ambifaria 188
α-cadinol 20, 22
calcium chloride 115
camphor 20, 21
captafol 214
captan, seed dressings 332, 332, 333
caraway oil 38
carbendazim 99, 226
carboxin, seed treatment 334
Carthamus oxycantha (Pohli weed) 269
Carthamus tinctorius, see safflower
carvacrol 20, 22
carvone 20, 21, 40, 53, 57
caryophyllene 20, 21
cash crops, mycorrhizae 174
castor 271–273
cedarwood oil 38
Cephalosporium gramineum stripe 238–239
Ceratocystis paradoxa 225
Cercospora arachidicola 263–264
Cercospora leaf spot
fenugreek 249–252, 250
sesame 271
Cercospora sesami 271
Cercospora traversiana 250–252, 250
cereal crops
mycorrhizae 172–173
rust diseases 206
seedborne fungi, treatment with plant
extracts 57
see also individual cereal crops
cerebrosides 187
charcoal rot, fenugreek 250
charcoal stump rot 174, 177
chemical fungicides 36, 183
drawbacks of 36, 62
Phytophthora sojae control 323
regulation 110
resistance 99–100
rice blast disease 99–100
root and butt rot of alpine forests
355–356
rust diseases 213–214
seed treatments 331–333, 332
storage diseases 16–17, 34
sugarcane diseases
pineapple disease 225–226
rusts 221
smut 219
tan spot 283
wheat 283, 292
Chenopodium procerum 40
chestnut blight fungus 123, 186
chickpea
mycorrhizae and fungal diseases 176
seed treatments 333, 334
chilli seeds 335, 335
chitinases 125, 127, 177
377
378
Index
chitosan 9
Cicer arietinum 209
1,8 cineole 20, 22
cinnamaldehyde 20, 21, 53
cinnamate derivatives 95
cinnamon oil 56, 58
citral 39, 53
citronellol 20, 21
citrus fruit 115
brown rot 4
essential oil treatments 44
green mould 115–116
root rot 174
Cladosporium spp. 212
Cladosporium herbarum, wheat leaves 239–241
Claviceps 54
Clavicipitaceae 150
Climacocystis borealis 347, 348
control 351–358
diagnosis 351, 351
climate
India 15, 37
and postharvest diseases 15
and powdery mildew of wheat 368–369
Septoria tritici blotch development
298–299, 298
climatic factors, seed storage fungi 340
clove oil 38, 56, 60
clover, rusts 207
cocoa beans, postharvest damage 29
coffee
rust diseases 204–205, 206, 208
fungicides 214
cold storage 15
collar rot
castor 272
fenugreek 250, 252
groundnut 265
Colletotrichum spp. 56, 186
Colletotrichum falcatum 221–222
Colletotrichum gloeosporioides 7, 154
common bunt (Tilletia laevis) 143–145
competition 112, 122–123
Trichoderma 127–128
‘compound interest diseases’ 205
containers, seed storage 336, 338, 340, 341
copper-based fungicides 214, 335, 340
corn smut 140–142, 143
cotton
mycorrhizae, effects on fungal diseases
176
rusts 207, 210
coumestrol 177
cowpea
mycorrhizae 173, 176
seed treatments 333, 334
Crambus spp. 188
crop residues, wheat 283
crop rotation
forests 353
Phytophthora sojae control 323
tan spot control 283
cross-protection 123
Cryphonectria (Endothia) parasitica 123
cryptocin 186
cucumber, mycorrhizae, effects on fungal
diseases 176
cultural practices
alpine forests 350
effects on mycorrhizae 163–165
fruit production 5
wheat diseases 365, 368, 369
Septoria leaf blotch resistance 74
tan spot 283
see also no-tillage systems
cumin 60
Cuminum cyminum 174
curcumene 20, 22
Curvularia spp. 56
Curvularia protuberata 190
cyanide (HCN) precursors 177
ρ-cymene 20, 21
Cyprus rotundus extracts 40
cytomegalovirus, human (hCMV) 187
Dalbergia rust 206
damping-off diseases
fenugreek 249, 252
sunflower 267
Trichoderma control products 133
defences, see plant defence responses
deformed plants 30
deoxynivalenol (DON) 80, 81, 83, 86, 364
inhibition by essential oils 58
recommended levels 364
Dichanthelium lanuginosum 190
diene antifungal compounds 4
discoloration of crops 29, 54
disease resistance
agronomic characters 73, 81–82
FHB 80–81
induced 9, 123–124, 128–129
morphological 80, 81–82
non-specific (partial) 95–96, 322
physiological, types of 80–81
race-specific 321–322
RBD 95–99
Septoria leaf blotch 70–73
see also endophytes, plant protection; plant
breeding; resistance genes
DON, see deoxynivalenol
downy mildew, sunflower 266–267
Drechslera tritici-repentis, see tan spot
Index
drought tolerance, and endophytes 189
drying of produce 33, 341
dryland crops 246
Dutch elm disease 188
early leaf spot, groundnut 263–264
egusi melon 61
endophytes
bacteria 150
beet leaves 151–152, 153
definitions 149–150, 183–184
ecological role and strategy 150–151
as gene vectors 151
groups 184
non-grass plants 150
plant protection 151, 159
abiotic stress 189–190
bacterial disease 186–187
fungal disease 159, 185–186
insects 188–189
nematodes 188
viral disease 187
potential of 190
research 151, 184–185, 190–192, 191
soybean leaves 154–155, 154
tomato leaves 152–154, 153
wheat 155–156, 157–158, 159
environmental conditions
and powdery mildew of wheat 368–369
seed storage 337–340, 341
Septoria tritici development 298–299
stored crops 31–33
enzyme-linked immunosorbent assays (ELISA)
299
enzymes
AM fungi 177
biological control agents 112, 125, 127
fungal pathogens 29–30, 32
epidemiology, definition 292
ergot 31, 54, 364
ergovaline 150
Erysiphe cichoracearum 270
Erysiphe polygoni 254–256, 257, 258
Escherichia coli, recombinant 125
essential oils 8–9, 18, 37–38, 43–44
active components 37, 53
biocide formulation 42
chemical structures 21–22
efficacy 19, 20
phytopathogenic fungi 38
seedborne fungi 38–39, 52–53,
55–61
toxin producing fungi 39
mode of action 44–45, 53
toxicity 45–46
estrobirulinas 283
379
ethanol 115–116
eugenol 20, 53
Eutypa dieback 9
expressed sequence tags (EST) 87
eye spot disease, sugarcane 223–224
fenchone 20, 21
fennel 60
fenugreek
abiotic disease 246–247
bacterial disease 247, 248
biology 245–246
crop potential 246
disease-resistant cultivars 257
fungal disease 249–256, 257
collar rot 250, 252
Fusarium wilt 252–253
leaf spot 250, 252
pod spot 250, 253
powdery mildew 254–256, 257, 258
spring black stem/leaf spot 253–254
insect pests 247, 248, 249
nematodes 247, 248
seed extracts, antifungal activity 249
seed treatments 334–335, 335
viral diseases 247
ferbam 213
fertilization 74, 369
fescue, tall 150, 187, 188
fescue toxicosis 150
FHB, see Fusarium head blight
fibre crops, rusts 207
fig (Ficus spp.), rust fungi 210–211, 214
fir, silver, root and butt rot 347
flavour, damage by fungi 29
flavour compounds, fruit 7–8, 42–43
fluzilazol propiconazol 283
Fomes annosum, see Heterobasidion annosum
Fomitopsis pinicola 347, 348
control 351–358
diagnosis 351, 351
food grains, loss in storage 329–330
food security 14
foot rot, black pepper 174
forage crops, rusts 207, 216
forest rotation 353
forests
role of mycorrhizae 174, 175
see also alpine European forests
free fatty acids (FFAs) 29–30, 331
fruit
antifungal compounds in unripe 4
aromatic and flavour compounds 7–8,
42–43
cultural disease control 5
disease-resistant transgenic plants 10
380
fruit continued
nutritional value 3
postharvest diseases 3, 4
integrated management 7, 115–116
plant extract treatments 7–9, 42–44
prevention 5–7
production in India 4
rust fungi 213
frutiafol 283
fumigation, botanicals 5, 42, 43, 44, 60
fumonisins 39, 80
fumonism 31
fungicides, see chemical fungicides
fusarenon-X (FUS-X) 80
Fusarium spp.
mycotoxins 80
plant extract treatments 57–59
postharvest pathogens 79
toxin production 31, 364
Trichoderma control products 133
Fusarium avenaceum 79, 80
Fusarium culmorum 79, 80
Fusarium graminearum 31, 32, 79, 80
Fusarium head blight (scab/FHB) 78–79,
363–364, 364
epidemics and crop losses 79
host resistance 80–81
barley 81–83, 87–88
wheat 83–87
mycotoxins 364
species isolated 79
symptoms and effects 79–80, 363–364
Fusarium moniliformae 224–225
Fusarium oxysporum 252–253
Fusarium oxysporum f.sp. ricini 272–273
Fusarium oxysporum f.sp. sesame 270
Fusarium poae 79, 80
Fusarium wilt
and AM fungi 173
castor 272–273
endophyte-induced resistance 185
fenugreek 250, 252–253
safflower 268
sesame 270
galactomannans, fenugreek 246
garlic extracts 56, 59, 60, 61
gene silencing 9
geothermal soils 190
geraniol 53
germination
and seed mycoflora 331, 331
and seed treatments 333–336, 333, 337
Gibberella moniliformis 224
Gigasporaceae 166
ginger 44, 56
Index
Gliocladium spp. 130, 270
Gloeosporium rot 5
Glomeromycota spp. 172
in no-tillage systems 163–164, 166–167
Glomosporium amaranthi, see Thecaphora
amaranthi
Glomus coronatum 188
beta-1,3-glucanases 125, 177
glucosinolates 8, 43
glume blotch 364
Glycine max, see soybean
gram (Cicer arietinum), rusts 209
grapes, postharvest technology 6
grapevine, Eutypa dieback 10
grass species
endophytes 150, 187, 188, 189
head smut 139–140, 140, 141
tan spot hosts 279
green mould, citrus fruit 115–116
green revolution, India 205–206
grey mould, strawberry 4
grey rot, Botrytis 272
groundnut 263
collar rot 265
early leaf spot 263–264
late leaf spot 264
postharvest damage 29, 30, 33
role of mycorrhizae 173
rust 203, 206, 208
fungicides 213, 214, 264
resistant varieties 264
seed and seedling diseases 264–265
growth abnormalities, damaged seed 30
guava 4
hairpin-encoding genes 99
halogenation, seeds 342
head blight, see Fusarium head blight (scab)
head rot, Rhizopus 267
head smut, Bromus spp. 139–140, 140
heading date 73, 81–82
heat tolerance, and endophytes 190
heat treatments 15–16
Alternaria leaf blight 268
fenugreek seed 253
fruit crops 5, 116
red rot of sugarcane 223
heating of crops (deleterious) 30
Helianthus annuus var. macrocarpus, see
sunflower
Helicobacter pylori 187
Helminthosporium carbonum 10
Helminthosporium oryzae 56
Heterobasidion annosum sensu lato 347, 348,
349, 350
control 122–123, 351–358
Index
diagnosis 351, 351
species susceptbility 353–354, 354
Heterosporium medicaginis 253
hevien 9
hexanal 42–43
hexenal 42–43
hinosan, seed dressings 332, 332, 333
honeybees 45
horticultural crops
mycorrhizae 173–174, 176
rust diseases 207
‘host shifts’ 94
host-specific toxins (HST) 223
hrf1 gene 99
hydration-dehydration treatments 342
hyperparasitism 123
hypersensitive response (HR), soybean 320
hypovirulence 123
immunoassays, Septoria tritici 299
India
castor production 271
climate 15, 37
endophyte research 190–192, 191
fruit production 4
green revolution 205–206
indole derivatives 186
induced resistance 9, 123–124, 128–129
insect pests
fenugreek 247, 248, 249
protective effects of endophytes 188–189
stored products 28–29, 31
integrated disease management 62, 124
fruit crop diseases 7
Phytophthora sojae 323
postharvest fruit disease 7, 115–116
postharvest fungi 115–116
root and butt rot fungi of trees 356–358
rusts 215
Septoria leaf blotch 73–74
ionizing radiation treatments 5–6, 16
iron 130
isoleucine 246
jasmonates 8, 43
javanicin 187
jowar, rusts 206, 208–209, 216
jute bags, seed storage 336, 338
karnal bunt (partial bunt) 366–368, 367
Laetiporus sulphureus 347, 348
Laetisaria arvalis 284
381
larch (Larix), root and butt rot 347, 353, 354
late leaf spot, groundnut 264
latex 9
leaf blight
Alternaria carthami 267–268
Alternaria infectoria 234, 241
Alternaria triticina 232–233, 241
leaf blotch (Septoria) 70–74
leaf rust, wheat 364, 365–366
leaf spot
Alternaria 265–266, 270
Ascochyta 236–237, 252
Cercospora, sesame 271
groundnut 263–264
Phoma sorghina 237–238
Pyricularia grisea 241
lectins 127
legume crops
mycorrhizae 173
rust diseases 209–210
fungicide treatment 214
seed treatments 333, 334
lemongrass oil 39, 58
leucine-rich repeats (LRRS) 212
Leveillula taurica 270
Lewia infectoria 235–236
lime-sulphur 213
limonene 20, 21, 53
Limonomyces roseipellis 284
linalool 20, 21, 53
lineage exclusion hypothesis 98
linseed 207, 209
lipases 29–30, 127
logging, impact on root and butt rot fungi 350
lolitrem B 150
Lr genes 366
Luffa acutangula 340
Lycopersicon esculentum, see tomato
lyso-phosphatidylcholine 175
Macrophomina phaseoli 29
Macrophomina phaseolina 272
Magnaporthe graminicola 293
Magnaporthe grisea 92–93
pathotypes 93–95, 97
maize
postharvest damage 30
rusts 206, 209, 216
seed spoilage in storage 330
seedborne fungi, plant extract treatments
58
smut 140–142, 143
mancozeb 213, 333, 333
maneb 213
manganese 130
mango 4
382
marker-assisted selection (MAS)
soybean 323
wheat 86–87, 88, 369
medicinal plants
fenugreek 246
northern India 185, 190
rusts 207
Meloidogyne incognita 188, 247
melon seeds, plant extract treatments 41, 61
menthol 20, 22
menthone 20, 21
mercury fungicides 214
methyl bromide 183
methyl jasmonate 43
methyl salicylate 46
MGR586 DNA repeat element 94
microtubules 177
minerals, solubilization and sequestration by
Trichoderma 128–129, 130
moisture, stored crops 31–32
molecular diagnostics, root and butt rot fungi
351, 351
molecular markers
Phytophthora sojae 321
soybean 323
wheat 86–87, 369
moringa 41
morphological disease resistance 80, 81–82
mulberry 206
mung bean 173, 333, 334
muskmelon seed 334, 335
mustard 334
mycoparasitism 123, 127
mycoparasitism related genes (MRGs) 126
mycorrhizae 124
soil infectivity 164
see also arbuscular mycorrhizal fungi
(AMF)
Mycosphaerella arachidis 263–264
Mycosphaerella graminicola 69, 70, 231
mycotoxins
aflatoxins 15, 29, 31, 39
control of production 39
DON 58, 80, 81, 83, 86, 364
Fusarium head blight 80, 364
mould species producing 32
safe limits 34
myrcene 20, 21
Myrothecium roridum 284
nabam, rust diseases 213
neem extracts 6, 8, 34, 40, 41
neem oil 42, 56, 60
neem seedlings, AMF 174, 175
neembicidine 333
nematodes
Index
and endophytes 188
fenugreek 247, 248
and mycorrhizae 176–177
nitrogen fertilization 74, 369
nivalenol (NIV) 80
no-tillage systems
and arbuscular mycorrhizae 163–164,
166–167
problems of 163
soil property changes 163
and tan spot control 283
wheat crops 74, 276
Norway spruce, root and butt rot 347, 352, 353,
354, 354
notchi powder 41
nucleotide-binding site plus leucine-rich repeat
(NBS-LRR) genes 98
nutrients
competition for 112
see also plant nutrients
nutritional requirements, fungi 32
oat, rust 206
oats 283
ochratoxin 31
odour changes 29
Oidium erysiphoides 270
oils
fruit skin coatings 6
see also essential oils
oilseed crops
mycorrhizae 173
rust diseases 206
seed treatments 333, 334
see also individual oilseed crops
olive oil 38
onion, mycorrhizae 176
onion-pink rot 173
Onnia tomentosa 347, 348
oocydin 186
oranges 44
orchard hygiene 5
oregano oil 38, 60
ornamental plants, rusts 207
orthodihydroxy (O-D) phenols 177
oxanthiin-carboxin, rusts 214
oxycarboxin 214
oxygen 32
ozone treatments 5
paddy, seed treatments 334, 335, 335, 336
palm oil 30, 33
palmarosa oil 58
papaya 5, 9, 41
pawpaw 40
Index
PCR-based diagnosis, root and butt rot fungi
351, 351
pea, rust 209, 213
pearl millet
rust diseases 206, 209
seed spoilage/treatments 330, 335–336,
336, 337, 338
Penicillium, plant extract treatments 59–61
pepper, mycorrhizae 176
peppermint oil 38
Peronospora trifoliorum 249
peroxidase 129, 177
Pestalotiopsis microspora 186
Phaeoisariopsis personata 264
Phaeolus schweinitzii 347, 348
phalsa 174, 213
phenylalanine 177
Phlebia gigantea 122–123
Phoma spp., Trichoderma control products 133
Phoma pinodella 249, 253–254
Phoma sorghina 237–238
Phomopsis oblonga 184
Phomopsis psidii 4
physiological specialization
Phytophthora sojae 320–321
powdery mildew of wheat 368
rice blast fungi 93–95, 97
tan spot on wheat 281–282
Tilletia laevis 143–145
Ustilago bullata 139–140, 141, 142
physiology of resistance, rice blast 95
phytoalexins 95, 129
in AMF-containing plants 177
induction by yeast antagonists 112
Phytophothora spp., Trichoderma control
products 133
Phytophthora drechsleri 268
Phytophthora infestans 185
Phytophthora nicotianae var. parasitica
173–174
Phytophthora parasitica var. sesame 269
Phytophthora sojae 319
life cycle 319–320, 319
physiologic races 320–321
Phytophthora spp., blight of sesame 269
Pi-ta gene 98
Picea spp., root and butt rot 347, 349, 353, 354
pigeon pea blight 173
pine oils 53
pine, Scots, root and butt rot 347
pineapple disease, sugarcane 225–226
α-pinene 20, 21
Pinus spp., root and butt rot 347, 349, 353, 354
plant breeding
Fusarium head blight (FHB) resistance 80
powdery mildew resistance 257
rust disease resistance 212
383
Septoria leaf blotch resistance 71–73
wheat 86–87, 363, 369
plant defence responses 95, 123–124
effects of mycorrhizae 175–176
provocation by Trichoderma spp. 304–305
plant extracts 39–40
effective against phytopathogenic fungi
39–40
effective against seed fungi 40–41
fruit crop treatments 44
total number 37
see also botanicals; essential oils
plant growth
and mycorrhizal associations 172
and Trichoderma spp. 129–130
plant height, and disease resistance 73
plant nutrients
and fruit storage rot 5
solubilization/sequestration by
Trichoderma 128–129, 130
uptake and mycorrhizal associations
176–177
plantation crops
rust diseases 206–207
see also alpine European forests; forests
plantavax w.p. 214
plantibodies 9
Plasmopara halstedii 266
Plasmopara patens 266
Plasmophara perennis 266
Plebiopsis gigantea 355
pod spot, fenugreek 253
Pohli weed 269
pokkah boeng disease 224–225
polyphenol oxidase 177
polythene bags, seed storage 336, 338
population genetics 292–293
postharvest diseases 14–15, 28, 29–30, 54–55
biochemical effects 29–30
conditions favouring 31–33
crop losses 109
crop weight loss 30
discoloration of crops 29, 54
flavour and odour changes 29
fruit crops 4–5
botanical as antifungal agents 7–9
integrated control 115–116
management 5–7
Fusarium spp. 79
growth abnormalities 30
insects 28–29, 31
management 33–34
biocontrol products 110
botanicals 17–18, 19–22
preparation for attack by other agents
30–31
rotting and caking 30
384
postharvest diseases continued
see also seed storage; storage diseases
potassium metabisulphite 341
powdery mildew
fenugreek 249, 250, 254–256, 257, 258
plant extracts 38
sesame 270–271
wheat 368–369
predation 124
preservatives, chemical 34
prochloraz 283
propagative materials, prevention of storage
damage 34
propionic acid 341
proteases, apoplast 304
Pseudonmonads 124
Puccinia arachidis 264
Puccinia graminis f.sp. tritici 364, 365
Puccinia helianthi 266
Puccinia kuehnii 220–221
Puccinia melanocephala 220–221
Puccinia striiformis f.sp. tritici 366
Puccinia triticina f.sp. tritici 365–366
pulegone 20, 21
pulses
mycorrhizae 173
rust diseases 207, 209–210, 216
pungam 41, 42
Pyrenophora tritici-repentis, see tan spot
Pyricularia grisea 241
Pyricularia oryzae 186
Pythium spp.
damping-off 249
Trichoderma control products 133
Qfhs.ifa-5A QTL 84
QFhs.ndsu-3BS QTL 84–85
Qrgz-2H-8 gene 83
QTL, see quantitative trait loci
quantitative trait loci (QTL)
Fusarium head blight resistance 78, 82,
84–87
Phytophthora sojae resistance 323
Septoria leaf blotch resistance 71, 72
‘quelling’ 10
race typing, Ustilago scitaminea 218
radiation, stored food commodities 5–6, 16
Radopholus similis 188
rainfall
and Septoria tritici blotch development
298–299, 298
and storage fungi 340
rainforests, mycorrhizae 175
random amplification of polymorphic DNA
(RAPD) 95–96, 321
raspberries 8
Index
RBD, see rice blast disease
reactive oxygen intermediates (ROI) 95
reactive oxygen species (ROS) 190
recombinant inbred lines (RILs) 85, 323
red rot, sugarcare 221–223
red smudge 278
relative humidity (RH)
seed storage 337–339, 339
Septoria tritici blotch development 298, 298
stored crops 31–32
repeat induced mutation (RIP) 10
repeated DNA sequences, Magnaporthe grisea
93–95
resistance gene analogues (RGA) 99
resistance (R) genes 10
FHB 82–83
Hm1 10
race-specific 321–322
rice blast disease 96–99, 97
rusts 212
soybean 322–323
wheat 366
resorcinols 4
Reynoutria spp. 38
RFLP techniques 301
rhizobacteria-induced systemic resistance (RISR)
128
Rhizoctonia solani 133, 249, 250, 252
rhizome rot, ginger 174
Rhizopus arrhizus 267
Rhizopus nigricans 267
Rhizopus oryzae 267
Rhizopus stolonifer 5
Rht-D1 locus 86
rice
postharvest damage 30
seed fungi 55, 57
sheath rot 42
rice blast disease (RBD)
control 99–100, 100, 186
crop losses 92
epidemiology 93
fungal agent (Magnaporthe grisea) 92–93
races 93–95, 97
fungicide resistant 99–100
host resistance
genetics 95–99
non-specific 95–96
physiology 95
symptoms 93
Ricinus communis, see castor
RISR, see rhizobacteria-induced systemic
resistance
ROI, see reactive oxygen intermediates
root and butt rot fungi, biological and chemical
control 122–123, 354–356
root knot nematodes 176–177
Index
root rot
Macrophomina 250, 272
Phytophthora 268–269
safflower 268–269
wheat (S. rolfsi) 172, 177
ROS, see reactive oxygen species
roses, rusts 210
rubber plant 174, 177
Rumex crispus 40
rust fungi 204
bean 209–210
coffee 208
cotton 210
epidemics and losses from 204–205
fenugreek 249, 250
fig 210–211, 214
grain crops 206
gram 209
groundnut 203, 206, 208, 209, 264
key to species/varieties 216
linseed (flax) 209
maize 206, 209, 216
management strategies 211–215
orange 220
pea 209
pearl millet 206, 209
rose 210
safflower 269
Sorghum spp. 206, 208–209, 216
soybean 210
sugarcane 220–221
sunflower 266
systematics 201–204
teleutospores 204
urediniospores 203–204
wheat 207–208, 364–366, 364
rye, rusts 206
ryegrass, endophytes 187, 188, 189
‘ryegrass staggers’ 150
safflower 267–269
Alternaria leaf blight 267–268
Fusarium wilt 268
Phytophthora root rot 268–269
rust 206, 213, 214
Salvia officinalis 38
sambangi 41
savoury oil 58
sclerotia 33, 54
Sclerotinia spp., Trichoderma control
products 133
Sclerotinia sclerotiorum, endophyte protection
185
Sclerotium rolfsii 172, 177, 265
stem rot 265
Trichoderma control products 133
385
Scots pine, root and butt rot 347, 353, 354
seed abortion 54
seed extracts, disease suppression 249
seed health 329
seed necrosis 54
seed piece infection, pineapple disease 226
seed storage
containers 336, 338, 341, 342
postharvest strategies 341
seed treatments
chemical fungicides 331–333, 332
fenugreek 251–252
persistence during storage 339–340
plant extracts 39, 40–41
essential oils 38–39, 52–53
Trichoderma spp. 129, 134
and viability 333–336, 333, 337
seed viability
loss in storage 30, 330, 331–336, 331
and seed treatments 333–336, 333, 337
and storage environment 337–340, 341
seedborne fungi
Fusarium spp. 79
management, plant extracts 40–41, 55–61
symptoms of disease 54–55
seeds
artificial 342
drying 341
free fatty acid content 29–30, 331
invigorating treatments 342
pelleting 342
Septoria leaf blotch
causal agent 70
crop yield losses 70
integrated management 73–74
resistance 70–73
Septoria tritici blotch (STB) 293
ascendant movement of disease 299–301
biological control 303–305
early detection 299
epidemiological studies 293–298
impact of climate 298–299
population genetic studies 301–303
serine 177
Serratia marcescens 186
sesame 269–271
Fusarium wilt 270
Phytophthora blight 269–270
powdery mildew 270–271
Sesamum indicum, see sesame
sheath rot, rice 42
siderophores 130
skin coatings, fruit 6
smut 138–139
amaranth 311–312
characterization of pathogen 313–315,
314
386
Index
smut continued
amaranth continued
incidence in amaranth cultivars 315,
316
wild hosts 313, 315–316
head smut, Bromus spp. 139–140, 140, 141
maize 140–142, 143
sugarcare 218–219
‘smut whip’ 219
sod webworms 188
sodium bicarbonate 115–116
sodium bisulphate 5
sodium metabisulphite 341
soils
acidity 189
diseases control 124–125, 125
mycorrhizae propagule banks 163, 164–167
no-tillage systems 74, 163–164, 166–167,
283
sorghum, seed treatments 335
Sorghum spp., rusts 208–209, 213
sowing time 249
soybean
endophytic fungi 154–155, 154
mycorrhizae 173, 177
Phytophthora root/stem rot 318
crop losses 318
disease cycle 319–321
management 321–323
pathogen 319, 320–321
symptoms 319
rust fungi 206, 210
seed treatments 333, 333
Sphaerellopsis filum 212
Sphaerotheca fuliginea 270
spike morphology 81–82
spinach, seed treatments 334, 335
sponge-gourd 41
spring black stem, fenugreek 253–254
spruce, Norway, root and butt rot 347, 352, 353,
354, 354
star anise 56, 60
stem rot, S. rolfsi 265
stem rust (black rust), wheat 364, 365
Stereum sanguinolentum 347, 349, 350
control 351–358
diagnosis 351, 351
steroidal sapogenins 246
storage diseases
common fungal species 330
conditions favouring 31–33
management 15–17, 33–34, 340–341
botanicals and plant extracts 40–41
postharvest strategies 341
preharvest conditions 340
storage environment
fruit crops 33, 34
seeds 337–340, 341
strawberries 4, 5, 8
Streptomyces 124–125
stripe, Cephalosporium gramineum 238–239
stromatization 54
stylar end rot 4
sugarcane
economic importance 217
eye spot disease 223–224
pineapple disease 225–226
pokkah boeng disease 224–225
red rot disease 221–223
rust diseases 220–221
smut diseases 218–219
sulphur 213
sulphur dioxide 5
sunflower 265–267
Alternaria leaf spot/blight 265–266
downy mildew 266–267
head rot 267
rusts 206, 213, 214, 266
Suryanarayanan, Prof. T.S. 190, 191
sustainable disease management
rice blast 100, 100
see also biological control; integrated
management
T-muurolol 20, 22
take-all 172, 364
tamarind 41
tan spot 231–232, 241, 276
crop losses 276–277
disease cycle 278–280
management 159, 282–284
pathogen 277–278
physiological specialization 281–282
prevalence and range in South America
276
symptoms of infection 278
tannins 40
tea tree oil 53
tebuconazol 283
teliospores 219
temperatures, seed and crop storage 32,
337–339, 339
Terminalia ivorensis 175
terpenoids 44
terpine-4-ol 20, 21
Thecaphora amaranthi 311–312
Thecaphora amaranthicola 312
characterization 313–315, 314
incidence in amaranth cultivars 315, 316
in wild amaranth species 313, 315–316
thermotherapy
see heat treatments
red rot of sugarcane 223
Index
thiram
rust diseases 213
seed dressings 332, 332, 333
thujone 20, 21
thyme oil 9, 19, 38, 44–45, 56
thymol 20, 21, 44–45, 53
tillage
effects on mycorrhizae 163, 164–166
see also no-tillage systems
Tilletia indica 366–368, 367
Tilletia laevis (common bunt) 143–145
time of sowing 249
tobacco 174, 177
tomato
endophytes 152–154, 153, 185
mycorrhizae 173–174, 176
toxic metabolites 15, 29, 31, 39
Alternaria 232
Fusarium head blight 80
mould species producing 32
toxicosis, endophytes 150
transgenic plants, disease-resistant 10
trenches, isolation 352
triazoles, systemic 283
Trichoderma spp. 123, 125–126
chemicals produced 129
commercial use 131
compatibility testing 132–134
delivery methods 134
mass production and formulation 130–131,
132, 134
mechanisms of action 126–129
pesticide susceptibility 130
plant growth promotion 129–130
range of biocontrol uses 126, 126
red rot of sugarcane 223
safflower wilt 268
Septoria tritici blotch 303–305
solubilization/sequestration of plant
nutrients 128–129, 130
tan spot control 284
Trichoderma harzianum 126, 223, 265, 270,
273, 284, 303–304
mass production and delivery 134
Trichoderma viride 126, 265, 270, 273
trichothecenes 80
Type A 80
Type B 80
Triticum spp., see wheat
Triticum dicoccoides 87
Triticum macha 87
γ tubulin 177
‘tulsi’ 8
UG99 (stem rust) 365
urea treatments 355–356
387
urediniospores, Puccinia 221, 269
Ustilago bullata 139–140, 140, 141
Ustilago maydis 140–142, 143
Ustilago scitaminea 218–219
Ustilago tritici 364
UV illumination 5–6
vegetable crops
endophytes 185–186
mycorrhizae 173–174, 176
rust diseases 207, 209–210, 216
seed treatments 334–335, 335
verbenol 20, 22
verbenone 20, 22
Verticillium spp.
antagonistic effects of endophytes 185–186
parasitic on rusts 212
protective role of AMF 173
viral diseases
endophyte plant protection 187–188
fenugreek 247
vitavax 214
vomitoxin, see deoxynivalenol (DON)
Vrs1 locus 82
warehouse conditions 33, 34, 337–340, 341
water requirements, fungi 32
weed hosts, rice blast disease 94
wheat
abiotic stresses 363
Alternaria leaf blight 232–233, 241
Ascochyta leaf spot 236–237
black point/leaf blight (A. infectoria)
234–236
breeding for disease resistance 363, 369
MAS 86–87, 369
Cephalosporium gramineum 238–239
Cladosporium herbarum 239–241
common bunt (T. laevis) 143–145
endophytic fungi 155–159, 156–158
fungal diseases 69–70, 277, 363–369, 364
pathogen-specific thresholds 292
Fusarium head blight 363–364, 364
crop losses 79
resistance 83–87
global demand and production 69, 275,
291–292, 362–363
karnal bunt (T. indica) 366–368, 367
Phoma sorghina leaf spots 237–238
powdery mildew 368–369
Pyricularia grisea 241
root rot 133, 172, 177, 265
rust diseases 204, 207–208, 364–366, 364
fungicides 213, 214
leaf rust 364, 365–366
388
Index
wheat continued
rust diseases continued
stem rust 364, 365
yellow (stripe) rust 366
seed disease, plant extract treatments 57–58
seed dressings 333–334
Septoria leaf blotch 70, 71–73
Septoria tritici blotch (STB) 293–305
ascendant movement 299–301
biocontrol 303–305
early detection 299
epidemiological studies 293–298
impact of climate 298–299
population genetic studies 301–303
Sumai 3 cultivar 85–86, 87
tan spot 231, 241, 276
crop losses 276–277
disease cycle 278–280
management strategies 159, 282–284
pathogen 277–278
physiological specialization
281–282
prevalence and range in South
America 276
symptoms of infection 278
wilt, Fusarium
and AM fungi 173
castor 272–273
endophyte-induced resistance 185
fenugreek 250, 252–253
safflower 268
sesame 270
wintergreen, oil of 46
wood decay fungi 348
wrappings, fruit 6
xanthan gum 114
Xanthomonas alfalfa 247, 248
Xylaria sp. 187
yams 30, 34, 40
yeasts (biocontrol agents) 111
constraints in commercial development
113–115
integration with other control measures
115–116
mechanisms of action 111–113
yellow rice disease 31
yellow rust, wheat 364, 366
Yield-PlusTM 110, 114
Zea, see maize
zearalenone (ZEN/F2-toxin) 80
zero-tillage, see no-tillage systems
zineb, rust diseases 213
zingiberene 20, 22
ziram 213